Reporting from New York - Roger Federer is five inches shorter and about seven years older than Juan Martin del Potro.

But the 6-foot-1, 28-year-old Federer has 15 major titles. He is in his 21st Grand Slam tennis tournament final. He is in his 17th final in the last 18 majors. And today Federer might become the first man since Bill Tilden in the 1920s to win six straight U.S. championships.

The top-seeded Federer beat fourth-seeded Novak Djokovic, 7-6 (3), 7-5, 7-5, Sunday in the second men's semifinal at the U.S. Open. In the first, the sixth-seeded Del Potro conquered the wounded Rafael Nadal, 6-2, 6-2, 6-2. Nadal, seeded third, has been dealing with a strained abdominal muscle for a month and could not withstand Del Potro's power.

But Del Potro, a 20-year-old Argentine, probably would be well-advised to not watch highlights of Federer's artistic and athletic semifinal triumph.

On the next-to-last point of the match, Federer hit what he called the best shot of his life. Djokovic was serving and seemingly in control of the point, standing at the net and watching for his just-hit lob to land out of Federer's reach.

Except Federer materialized at the baseline just when the lob landed. With his back to the net, Federer swung the racket between his legs -- and hit a clean winner past Djokovic. The crowd roared, and Federer's forehand return winner that followed on match point was anticlimactic.

"On those shots, you just say, 'Well done, too good.' What can you do?" Djokovic said. While Federer was still on the court, the point was replayed on the scoreboard video screens at Arthur Ashe Stadium, and Federer exclaimed, "That's unbelievable."

So are many of Federer's accomplishments. If he beats Del Potro he will tie Tilden (six straight U.S. championships from 1920-25) and William Renshaw (Wimbledon, 1881-86) by winning six consecutive titles at a single Grand Slam event. Only Richard Sears, who won the U.S. title seven times in a row beginning in 1881, would be ahead of Federer.

In his way is Del Potro, who at 6 feet 6 is all lanky arms and legs. When he gets all those moving parts in sync, the forehands bound off his racket.

To Nadal it must have sounded as if he were inside a popcorn popper -- puff, puff, puff, the tennis ball seeming to multiply as it whizzed past Nadal over and over.

Wearing a sleeveless black shirt, black shorts and black shoes, Del Potro looked like a stick of licorice, and his body often seems as flexible as one.

Del Potro is now 17-1 since Wimbledon, though he has never beaten Federer.

"This is part of my dream," he said. "I'm very close to doing it, but this moment now is so nice. I've always dreamed of this moment."

Nadal, 23, struggled at several stages of the tournament because of his abdominal injury and three times Sunday he bent over and clutched his side.

After refusing to speak about the injury as long as he was still in the tournament, Nadal said it had been a problem since a tournament in Montreal last month.

"I think I had some [swelling], a strained muscle," Nadal said. "I think during the two weeks here the strain has [turned] into a little bit of a rupture."

Nadal said the pain was a problem especially when he served. "I couldn't serve a little faster or change a lot of directions," he said. "I could only serve in the middle because if I served it outside, the abdominal kill me."

ESPN2 interviewer Darren Cahill mentioned to Del Potro that many fans might have been hoping for a Nadal-Federer final.

"I'm sorry," Del Potro said. "Tomorrow I will fight until the final point for you. If I win, if I lose, already I think this is the best moment of my life."

Federer, a new father of twin daughters, seems unfazed by all the glittery statistics.

"Right now I'm pretty relaxed," he said. "We'll see how it goes when the sun comes up. I'd like to keep this going. It would be great to get my first Grand Slam [title] as a dad."

diane.pucin@latimes.com

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