Mardy Fish isn’t easy to recognize these days. When I walked onto the Grandstand this afternoon, Fish and his opponent, Jan Hajek, were facing each other at the net, waiting for the coin toss. I knew, in theory, that Fish was the taller of the two, but I was still a little stunned to see what this 30-pounds-lighter version looked like in person.

The rail thin legs and skinny arms, the now-baggy shirt, the newly unearthed muscle definition: It all combines to make him appear taller and, at least in his body type, younger. Fish isn’t quite used to it himself. As the chair umpire tossed the coin, he was busy drawing in the string around his shorts, which still looked a little big on him.

“I feel like a completely different person,” Fish said with a touch of wonder in his voice after his eventual five-set win over Hajek. That starts with the person on court. “I can do so many things with my game that I could never do before.” Fish wasted no time showing one of those things off. Hajek, an undersized but pesky Czech baseliner ranked 82nd in the world, tried a drop shot on the first point.

Fish, never a speedster in his past life, was on top of it in a (relative) flash. Much later, after he’d gone down two sets to one, Fish brought the crowd to its feet and grabbed the momentum back for good by tracking down a hard-hit forehand in one corner and looping it all the way back crosscourt for a winner. That shot came at the end of a long rally on a hot day, and it required speed and balance that Fish might not have been able to summon even as recently as last year.

According to Fish, the changes in his life and self-confidence extend far beyond the court. Asked what he thought when he looked at pictures of himself from a couple of years ago, he said he looked “really different. I look at Stacey [his wife] and say, ‘What was your problem? Why didn’t somebody tell me that I looked like that.”

“I feel healthy,” he added, moving from the comic to the therapeutic. “A lot of it has to do with just sort of walking around the locker room, feeling confident. I want to set a precedent to the guys that I can play in the hottest stuff out there. I want to sort of put some sort of myth out there.”

Video   Can Nadal finally win U.S. Open? Aug. 24, 2010: Tiffany Simons and Harry Cicma preview the upcoming action at the 2010 U.S. Open on the men's side.

The mythic Fish was reborn after he was forced to have knee surgery because, as he admitted today, he was too heavy. That’s not too surprising, considering that he also confessed that his training meal on the road often consisted of a late-night pie from Domino’s.

What I’ve been wondering is, what took him so long, why did he wait until he was 28 to get with any kind of program? Fish offered no excuses, saying he made bad choices and essentially coasted on his talent from age 20 to 26. He also credited getting married, which was once the death knell for a player, with making him hungrier — perhaps literally — and more determined. Fish may be one of those rare tennis players who needs to win for someone other than himself.

It was a big part of his win today. Hajek, who had dictated play through the second and third sets, visibly slowed at the start of the fourth — even his first-serve speed dropped precipitously. “It’s a good feeling,” Fish said, “to wear down a player and know that all the hard work that you put in just paid off right there.”

Still, Fish came perilously close to losing what may have been the most important and pressure-filled match of his career, to a guy he should beat easily. For long stretches, Fish backed deep behind the baseline and was content to let the shorter Hajek run the show. If you didn’t know better, you might have thought that Fish was playing with something to lose, and he wasn’t quite ready for that feeling. You might have been right.

“This is a new position for me,” he said. “It’s new to have a lot of expectations, have a lot of people talking about you. It’s where we want to be, but I’ll have to get used to it.”

Now that this early scare is behind him, is Fish ready to do big things at this year’s U.S. Open? How seriously should we take him as a contender? His history would tell us that as soon as we start to believe, that's exactly when we should think twice about him. On the plus side, he’s clearly a much more mature person and player. His serve and backhand have always been weapons. And he’ll know he can go five sets and win. But there are two caveats.

Number one: Is Fish a little too mature to break through at this late date? Is he a little too willing to admit his flaws to himself? This is a laudable trait in any human other than a professional athlete. Today Fish said he believed he could beat anyone in three-out-of-five, but that he had never “shown” that he could do that.

That’s certainly true — he’s never reached a Slam semi — but it strikes me as almost too thoughtful, and doubt-full, for someone who is going to have to make himself win the types of matches he’s never won before. I’d like to be wrong about this.

Slideshow   The Week in Sports Pictures The Giants on top of the football world, getting ready for the London Olympics and more. more photos

The second caveat is more cut and dry: After his serve, does Fish have the game from the baseline to go all the way? As I said, he played pretty passively today, especially during his return games. More important, Fish didn’t show off the type of point-ending forehand that we know wins majors. He has tweaked this stroke a few times over the years, but it will never be a natural born killer. It’s still mostly a looped rally shot that, if it doesn’t sit up, also doesn’t penetrate through the court for many winners.

Fish has a good draw. One seed in his section, Baghdatis, was knocked off Tuesday, and another, Novak Djokovic, used up a lot of energy avoiding his own brush with an upset. Whoever his opponents turn out to be, Fish will have to play better than he did today to beat them.

He may be able to go five in any kind of weather, but no one takes home the U.S. Open title on fitness alone — you still have to win one of the first three sets. New body, new game, new expectations: That's a lot to recognize, let alone get used to, in a couple of weeks.

Also on NBCSports.com Beyond the Arc: No. 11 Mich. St. takes big step forward Chandler: Jeremy Lin was special even in 8th grade What NFC teams need this offseason  |  And AFC DeMarco: Dodgers could become a contender Follow us on Twitter: @NBCSports  |  And Facebook