NEW YORK — Roger Federer is five inches shorter than Juan Martin del Potro. Federer is eight years older than del Potro.

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

The top-seeded Federer beat the fourth-seeded Novak Djokovic, 7-6 (3), 7-5, 7-5, Sunday in the second men’s semifinal. 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, would probably be well-advised not to watch highlights of Federer’s artistic and athletic semifinal triumph.

On the second-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 hit a winner from between his legs. 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 giant scoreboards and Federer exclaimed, “That’s unbelievable.”

So are many of Federer’s accomplishments. If he beats del Potro to- day (4 p. m., Ch. 2) he will tie Tilden (six straight U. S. championships from 1920-25) and William Renshaw (Wimbledon, 1881-86) by winning six consecutive events at a single Grand Slam event. Only Richard Sears, who won the U. S. title seven times in a row beginning in 1881, is ahead of Federer.

In his way is del Potro, who at 6-foot-6, is all lanky arms and legs. When he gets all those moving parts in synchronicity, the forehands bound off his racket.

To Nadal, it must have sounded as if he was inside a popcorn popper — puff, puff, puff, the tennis ball seeming to multiply as it passed 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 is licoricebendable. 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 with a strained abdominal muscle and three times Sunday he bent over and clutched his side.

After refusing to speak about the abdominal injury as long as he was still in the tournament, Nadal said it has 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 maybe many fans had hoped for a Nadal-Federer final today.

“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 as a dad.”