I’m not sure why, but there’s no doubt about it, doubt has always kind of interested me.

For starters, I was always intrigued by the adage that doubt is the only human emotion that proves its own existence — “I doubt that (the concept of) doubt exists, therefore it exists.” Plus, a friend once confessed to me, “I have doubt on every stroke. Even if I hit a good stroke, I’m surprised if it goes in. Doubt destroys tennis.”

She had a curious point. After all, in the ‘70s, in the biggest sports boom ever on Planet Earth, millions jumped aboard the tennis bandwagon. But many soon doubted they could master such a challenging game, so they fled the sport in hefty droves.

At its core, tennis is a dance between doubt and certainty. So when I heard there was a celebrated new Broadway play by John Patrick Shanley simply titled “Doubt” that had collected a Pulitzer and four Tonys, I made sure I saw it while I was in New York for the U.S. Open.

Doubt, slight or severe, shadows us, an unwanted companion. At times it is a mere subtle voice giving us reason for sensible pause, and yet it can be a roar that paralyzes. Not surprisingly, many sports mantras call for fearless action. “Just do it,” “Go for it,” “Just win, baby,” — are all hardy antidotes to doubt’s cautionary siren. Then again, even the greatest among us — Jesus, Buddha, Galileo — have famously wrestled with uncertainty, only to boldly channel their doubts as they build compelling new faiths or craft cutting-edge mindsets.

So it’s little wonder that we humble athletes, who are merely playing a game, can tremble on break point, be intimidated by the imposing ringer atop the club ladder, or choke big time on the most routine of overheads. As for the pros, an outbreak of errant groundies is enough to set Anastasia Myskina into a free-fall of disbelief, self-abusive chatter and mockery. Few in sports have so mastered the art of the implosion.

With Elena Dementieva, doubt is all a matter of a single flawed stroke. Okay, Shaquille O’Neal is beyond clunky on a NBA free-throw line. But the serve is so central in tennis and Elena serving at crunch-time is a frightful misadventure few (especially Dementieva) should have to endure. Her futility shouts doubt.

In contrast, Jana Novotna’s infamous disbelief seemed to spring from a deep pool of melancholy. The Czech’s poignant ‘93 Wimbledon collapse enshrined her as tennis’ poignant poster girl for universal fallibility. Any of us poor souls who’ve blown a tasty opportunity or two can feel more than a little sympathy for the fallen challenger heaving and weeping on the Duchess’ shoulder — “I coulda been a champion.”

But Graf had other ideas that day, just as she did in the ‘99 French Open final, when she handed the far-too-full-of-herself Martina Hingis her comeuppance. For too long, the smug Swiss teen had merrily pranced along doubt-free in her own hermetically sealed (“I’m Martina Hingis and you’re not”) world of privilege and clueless innocence. But that summer day, linesmen, Graf and a fiercely unforgiving French crowd all conspired to create a brutal firestorm of doubt that, in one cataclysmic afternoon, stripped away a teen’s breezy sense of entitlement and infalability.

Doubt descends on the game in a thousand ways — injuries, losing streaks, the speculation of the media or simply in the drone of the dreary (“If it’s Tuesday, this must be Belgium”) demands of tour life. Site and surface matter. For instance, Andy Roddick is a formidable, full-of-swagger force on grass and hard courts. But on red clay — when he’s being taken down by a triple digit, French no-name dirtballer, he falls into an inexplicable daze — a man adrift.

A single, dreaded foe can also undermine belief. Vitas Gerulaitis scored wins over many a splendid opponent. But his ongoing failure against Jimmy Connors, prompted his delightfully self-depreciating boast, “Nobody beats Vitas Gerulaitis 17 times.”

Doubt is so pervasive in this game that the sport as a whole at times becomes engulfed in hissy-fits of uncertainty — most famously when there was the dud of an all-Czech Ivan Lendl-Miloslav Mecir U.S. Open final in ‘86, which was followed by the notoriously gloomy Sports Illustrated cover story “Is Tennis Dying?” A palpable “the-sky-is-falling” panic terrified the suits.

Of course, if doubt embraces tennis, so too there is plenty of certainty. And nothing radiates tennis confidence like Wimbledon. Never mind that it’s played on an absurdly arcane surface, the place oozes an intoxicating “been there, done that” air. Do display your homage, this is the Mother Church, the unquestioned epicenter of the known tennis universe.

Still, there are times when tennis certainty is based on illusion. Shanley claims, “If you feel certain, it’s an emotion, not a fact.” For example, generation after generation of young wannabes burst onto the scene brimming with a bubbly certainty; that naive blue-sky confidence that all but tells them a Grand Slam trophy is their birthright. Young wide-eyed Alexandra Stevenson informed us again and again she would win Wimbledon. (She didn’t and is now ranked No. 673). But not to worry, even well seasoned vets get ensnared in false confidence. Blinded by past glories, an over-the-hill Michael Chang trudged on and on based on the illusory hope that he would return to the very top.

Certainly, doubt can ultimately empower as it fuels inspiration, discipline and perseverance. Many will contend that doubt’s work is never done. Still, remarkably, a few rare athletic geniuses seem to transcend doubt. Lithe young Graf at her zenith would quick-step about the court, as if to say, “Let’s get on with it; I’ve got other things to do.” When Sampras unleashed his leaping overhead he was simply reminding us, “I’m a player apart.” These days there’s a sublime Swiss lad — Federer, I think, is the name — who has a penchant for composing seamless masterpieces. And even if you mount a significant challenge against tennis’ reigning artist-in-residence, he’ll more likely view it as a curious inconvenience to be flicked aside, than some sustained threat to fear.

Still, when it comes to the Open Era Nobel Prize For Dent-Free Tennis Confidence, no one touches James Scott Connors. Shanley informs us, “In ancient Sparta, whoever shouted the loudest won.” Well, Jimbo shouted and Jimbo won (a record 109 tournaments). Chest out, ‘tude in place, a domineering gait, he was the personification of loud certitude, the cocksure battler who mustered a seamless bravado that withered even the bold. Just ask Aaron Krickstein or Pat McEnroe.

Ironically, it was his archrival John McEnroe who had tennis’ most storied, angst-ridden, season of doubt. Okay, this wasn’t exactly the Apostle Paul on the road to Damascus, but just as his red-hot career was beginning to hit some speed bumps, he suffered a curious loss to Brad Gilbert that opened the floodgates of doubt. He wrote, “Someone is telling me something because if I can lose to Gilbert, something is seriously wrong. I’ve got to re-evaluate not only my career, but my life.” For eight months he took a sabbatical to tame his demons and to tune into his inner Mac. Forget “you’re the pits of the world”, new-Mac told us that “Tennis points may be inspiring at the moment, but then the moment is gone. They’re like poetry written on water.” An adopted Californian, he became enamored with the Pacific’s surf and sunsets and on an alluring Malibu beach had an epiphany during which he realized there was far more to life than tennis.

Surely any true fan knows this all too well. At the U.S. Open, James Blake — who just a year ago lost of his Dad, broke his neck and endured the partial paralysis of his face — told IT, “I wondered if my face would ever come back to normal. I tried to think about every situation and find a way to be happy. If I couldn’t play tennis again, was I going to be happy going back to school ... Would I be happy if my eye never came back to normal and I couldn’t play for the rest of my life?”

Certainly, Monica Seles, who at the formidable peak of her career was stabbed by a crazed Graf fan, confronted similar soul-shaking doubts. But more than any other tennis figure, it was Arthur Ashe — who suffered a heart attack, had brain surgery and succumbed to AIDS — who struggled with the pervasive doubts life and death struggles stir. The product of the segregated South, for him, emotional control was a survival mechanism. His old-school strategy was firm. “Despair is a state of mind to which I refuse to surrender,” he wrote. “I resist moods of despondency because ... they feed upon themselves. I fight vigorously at the first sign of depression. Some depression can be generated by the body rather than the mind...[and] is hard to contain. But depression caused by brooding on circumstances, especially circumstances one cannot control is another matter. I refuse to surrender myself to such a depression and have never suffered from it in my life.”

He continued, noting, “Here there are very close parallels between ordinary life and athletic competition. The most important factor determining success in athletic competition is often the ability to control mood swings that result from unfavorable changes in the score.

A close look at facial expressions and body language reveals that many individuals or even entire teams go into momentary lapses of confidence that often prove disastrous. The danger is that a momentary lapse will begin to deepen. Once in motion, it seems to gather momentum...A few falling pebbles build into an avalanche. The initiative goes to one’s opponent, who seems to be on a roll; soon, victory is utterly out of reach. In [my] life-threatening situation, I had to do everything possible to keep this avalanche of deadly emotion from starting. One simply must not despair, even for a moment.”

Strong stuff, but then again Shanley asserts, “We’re just ideas and convictions,” a sentiment which echoes Boris Becker’s iconic insight that “the fifth set has nothing to do with tennis, it’s all about the mind.” The true combatant is a master of doubt management. So police those free-form expressions: please no drooping shoulders (like Stefan Edberg) and, of course, no unnecessary sighs or hints of defeatism.

Athletic competition diverges dearly from intimate relationships where open, expressive, emotion and spontaneity are invaluable. On the march to victory the athlete draws from within — firm resolve, tested discipline — and exudes an aura of triumphant destiny. Here a passing expression of doubt or the most minute wavering, could tip the oh-so-delicate scale.

But the central figure in the play “Doubt” is no athlete. Instead, she’s a no-nonsense principal in an early ‘60s parochial high school in the Bronx; a fiercely certain authoritarian in a most authoritarian institution, who has just bent her beloved rules in order to emerge triumphant in an internal school battle. But in doing so, she has transformed her cozy universe of certitude into a brave new world of perplexing nuances without comforting restraints. Ultimately, she reflects on her troubling new fate and confesses to the audience, “I have doubts. I have such doubts.”