Juan Martin del Potro rallied from a two-sets-to-one deficit to defeat five-time defending champion Roger Federer 3-6, 7-6 (5), 4-6. 7-6 (4), 6-2 in the final of the U.S. Open yesterday. The 20-year-old Argentine, shaky and nervous in the beginning, served big when he needed to and pounded bazooka forehands that gradually wore Federer down in an ebb-and-flow contest that lasted four hours and six minutes. Federer was going for his 16th career Grand Slam title and third of the year after winning the French Open and Wimbledon earlier this season. No. 6 del Potro, while not a household name outside tennis circles, had his breakthrough last summer, winning 23 matches in a row and four straight hard-court tournaments. The 6-foot-6 Argentine, who administered an impressive beatdown of Rafael Nadal in the semis on Sunday, had lost his previous six matches against Federer. “Maybe I look back and have some regrets about it,” said Federer, never beaten by anyone other than Nadal in a major final. “But, you know, you can’t have them all and can’t always play your best.” Federer struggled with his serve throughout the match, committing 11 double faults and connecting on just 47 percent of his first deliveries. It didn’t matter at first as del Potro came out listless and uninspired. But the sleeping giant awoke in the 10th game of the second set, breaking Federer to even the set at 5-5. Del Potro won the tiebreaker and was in the match. Two straight double faults by the Argentine at 4-5 in the third set gave Federer the break and the set. Perhaps a little gun shy after the double doubles, del Potro took a little off his first serve over the final two sets and got a much higher percentage in. The tactic worked as Federer, uncharacteristically edgy and out or sorts, was most troubled by the 100 mph forehands del Potro was blasting at him. “When I would have a dream, it was to win the U.S. Open, and the other one is to be like Roger. One is done,” del Potro said during the on-court ceremony. Addressing Federer directly, del Potro added: “I need to improve a lot to be like you. I’d like to congratulate you for fighting ’til the last point.” The Argentine had more winners, 57 to 56, and committed fewer unforced errors, 60 to 62. Del Potro became the first player other than Federer, Nadal or Novak Djokovic to win a Grand Slam since Marat Safin won the Australian Open at the beginning of 2005.