NEW YORK: World number one Roger Federer began the US Open fortnight with a new top rival for his tennis throne in Andy Murray and ended the year's last Grand Slam with yet another formidable challenger.

Juan Martin Del Potro joined the No. 1 conversation with a shocking 3-6, 7-6 (7/5), 4-6, 7-6 (7/4), 6-2 victory over the Swiss star in Monday's US Open final, the 1.98 metre Argentine giant snapping Federer's five-year title run.

Del Potro, who turns 21 next week, jumped into fifth in the latest ATP rankings by winning his Grand Slam final debut.

Federer remained first despite the loss with Spain's injury-hampered Rafael Nadal moving back into second on the list past Murray, who stumbled badly in a fourth-round loss to Croatia's Marin Cilic, and Novak Djokovic placed fourth.

"I have very good players from me to the No. 1," Del Potro said. "For sure, Roger, Rafa, Murray, they are two or three steps more than me. I'm working hard every day hard to be like them.

"But if I play for many years in this level, then they are going to retire and I'm going to be much better."

Time is not all that is on the South American's side.

His formidable forehand and improved fitness caught the eye of 15-time Grand Slam winner Federer when he was forced to rally from two sets down to beat him in a French Open semi-final.

"Every month he was getting better," Federer said. "He was hitting his serves better, from the baseline he was getting more confident. He knows much more now what he needs to do on the tennis court.

"Whereas maybe in the beginning, because he's so tall, he needs to figure out his game, I think he did that in the last six months."

Del Potro sees himself as far from a finished product, but it's bad news for every rival if he can only get better.

"I have everything to learn," Del Potro said. "I want to be a good player in the future. I want to be like them. I'm very happy I'm on my way."

With Nadal nursing strained stomach muscles and already out two months with sore knees and Murray turning in a miserable showing to help clear Del Potro's path to the trophy, the door is open for the big man to climb up the rankings.

Del Potro had lost all six prior meetings with Federer, including a five-set thriller in the French Open semi-finals, but became the only man other than Nadal to defeat Federer in a Grand Slam final.

"I just go my way and try to improve every day my game," Del Potro said. "Of course I'm living much better than two years ago, maybe 12 months ago. But I'm in good shape. I'm physically feeling good with my tennis. That's important."