Here we sit, on Tuesday, and there is just one undefeated player at the Masters Cup. He’s from Spain. Like a pit-bull, he’s tenacious and fast. He is not, however, named Rafa Nadal.

But David Ferrer’s impressive start in Shanghai – barring a disaster, he will reach the semifinals—should come as no surprise. The 5-foot-9 grinder has quietly put together one of the best records this year. Call him the sleeper hit of ’07. The 25-year-old, who is ranked No. 6, has won three titles this year – Tokyo, Bastad, and Auckland – and reached the semifinals of the U.S. Open. He’s beaten Nadal the last two times they’ve played, at the Open and today in Shanghai.

Aside from being rivals and countrymen, Ferrer and Nadal have something else in common: They are the only ATP players this year to win at least 20 matches on clay and 20 matches on hard courts.

Watching the hard-charging Ferrer as he jets around the court to track down would-be winners and force his opponents to hit one more shot, you’d never guess that this is the same player who once quit tennis because he lacked the fire inside.

Back in 1999, when he was 17, Ferrer was training in his hometown of Javea, with his coach, Javier Piles. The tennis gene was strong in Ferrer’s family—his older brother, Javier, was Spain’s 12-and-under champion; he’s now a coach—but Ferrer, like any teenager, wasn’t getting big results and, more importantly, couldn’t quite figure out if tennis was his calling. Hanging out with friends, on the other hand, suited him just fine. Predictably, his desire fizzled. That’s when Piles tried to motivate Ferrer with tactics straight out of a Saw movie.

He locked him in a closet.

“When he didn’t want to work I would lock him up in a dark room of two by two meters and I would put a lock on it so he couldn’t get out,” Piles told Deuce magazine. “It was the room where we would store the tennis balls. I would tell him that his working scheduled was from 9 to 12 and that if he didn’t want to work he would remain there punished. I would give him a piece of bread and a bottle of water through the bars of a small window. After a few minutes we would hear David asking other trainers from the club for some help to get out but we wouldn’t pay any attention.”

It was enough to send Ferrer running. He quit tennis, but—and you have to love the logic of a teenager—still wanted to go out and party. That requires money, of course, so Ferrer got a job in construction loading bricks. But one glance at his meager restitution – a paycheck of 30 Euros for the week – and suddenly, shockingly, Ferrer rediscovered his love for tennis.

And now here he is, in Shanghai among the world’s Top 8 players. Ferrer doesn’t have that big knockout punch. Or at least that’s the book on him. But against Nadal yesterday, he managed to hit 23 winners to Nadal’s 10. And talk about a role reversal for Nadal—he was the one, not his opponent, doing most of the running.

“If I don’t play perfect, I cannot beat Rafa,” a gracious Ferrer said after the match.

While Rafa is as singular as they come, with his acute angles and lefty game, Ferrer nicely fits into the Spanish mold of Tommy Robredo and Juan Carlos Ferrero. In other words, guys who’ll run all day long to wear down their opponents. Not the most scintillating style of play, to be sure, but one that’s to be appreciated against players who force the issue on the court.

Given Ferrer’s late-blooming career, and his grinding style, it’s hard to see him sustaining his high level of play. So enjoy watching him while you can. It’s not every day that someone who once described himself as the “worst player in the Top 100” reaches the season-ending championships.

Then again, it’s not everyday that someone is locked in a closet, either. It’s still there at the club, he says, now filled with balls. No doubt Ferrer hopes it stays that way.