NEW YORK – Always so cool, so consistent, so in control of his emotions and his matches, Roger Federer let the U.S. Open championship slip from his grasp.

Two points from victory against inexperienced, unheralded Juan Martin del Potro of Argentina, two points from a sixth consecutive title at Flushing Meadows and a record-extending 16th Grand Slam title overall, Federer fell apart Monday.

He railed at the chair umpire. His legs grew weary. His double-faults mounted. He could not figure out a way to stop the 6-foot-6 del Potro from pounding forehand after forehand past him.

In a result as shocking for who lost as how it happened, the sixth-seeded del Potro came back to win his first Grand Slam title by upsetting No. 1-seeded Federer, 3-6, 7-6 (5), 4-6, 7-6 (4), 6-2.

“Maybe I look back and have some regrets about it,” said Federer, never before beaten by anyone other than Rafael Nadal in a major final. “But, you know, you can’t have them all and can’t always play your best.”

He had won 40 straight matches at Flushing Meadows. He had won 33 of his previous 34 Grand Slam matches. And he has made the final at 17 of the past 18 Grand Slam tournaments, 21 overall.

“Five (in a row) was great, four was great, too, and six would’ve been a dream,” Federer said. “Can’t have them all.”

Del Potro? This was the 20-year-old’s first Grand Slam final, and he was 0-6 against Federer. But after handing Nadal the most lopsided loss of his Grand Slam career in the semifinals Sunday, del Potro came back the next day and rattled Federer.

“I would like to congratulate Juan Martin on an unbelievable tournament. I had a great one myself, too,” Federer said, “but he was the best.”

That’s some compliment.

Somehow, del Potro never seemed intimidated by the setting or the man many consider the greatest tennis player in history.

“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.

The usually unflappable Federer argued with chair umpire Jake Garner during a changeover, using a profanity and saying, “Don’t tell me to be quiet, OK? When I want to talk, I talk.”

He also got steamed while up a set and serving at 5-4 in the second. Del Potro tried a forehand passing shot that was called wide, but he challenged, and the replay system showed he was right. Federer kept glancing at the mark the shot left on the blue court, even into the next game, and del Potro wound up stealing the set.

“That one cost me the match, eventually,” Federer said.

Del Potro managed to have the time of his young life, high-fiving front-row fans after winning one point, and reveling in the soccer-style serenades of “Ole!”

“I did my dream and it’s unbelievable moment,” del Potro said. “Everything is perfect. I don’t know if I can explain. It’s my best sensation ever in my life. It’s too early to explain. Maybe tomorrow, maybe next week, I will believe in this. The crowd was so exciting, so respectful of both players. They made a good show for everyone.”

The 4-hour, 6-minute match was the first U.S. Open final to go five sets since 1999.

“I thought I had him under control for the first two sets,” Federer said. “But I thought Juan Martin played great, and in the end, he was just too tough. It was great to see him so happy and emotional about it. He should enjoy it.”

Serena apologizes, wins

Her doubles championship brought some closure to what has been an awkward, tumultuous U.S. Open for Serena Williams.

Williams and sister Venus teamed up for a 6-2, 6-2 victory over defending champions Cara Black and Liezel Huber to win their 10th Grand Slam doubles title, and first at Flushing Meadows since 1999.

“I just really wanted to apologize sincerely because I’m a very prideful person and I’m a very intense person and a very emotional person,” she said.

“Most of all, I think I’m a very sincere person. I wanted to offer my sincere apologies to anyone that I may have offended.”

The people offended, Serena said, were “the lineswoman, Kim Clijsters, the USTA and, mostly, tennis fans everywhere for my inappropriate outburst.”

During a post-match interview on the court, ESPN2’s Patrick McEnroe prodded Serena about her profane outburst at the end of her loss to Clijsters and its aftermath. Serena had issued an apology shortly before the doubles match, saying she is “a woman of great pride, faith and integrity, and I admit when I’m wrong.”

An investigation is being conducted to see if further sanctions will be instituted, which could cost her the $350,000 she won for reaching the singles semifinals, plus a possible suspension.