NEW YORK (AP) — Always so cool, so consistent, so in control of his emotions and his matches, Roger Federer amazingly 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, quite simply, 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 the 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 consecutive 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.

Del Potro? This was the 20-year-old’s first Grand Slam final, and he was 0-6 against Federer until now. 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.

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, meanwhile, 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!” ringing through the stadium.

“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.

Then, 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 4-hour, 6-minute match was the first U.S. Open final to go five sets since 1999, and there were no early signs to indicate it would be this competitive — much less end with del Potro down on his back, chest heaving, tears welling, a Grand Slam trophy soon to be in his arms. He is the fifth-youngest U.S. Open champion and the first man from Argentina to win the event since Guillermo Vilas in 1977.

Vilas was in the stands Monday, sitting one row behind Jack Nicklaus.

One simple indication of the difference in age and status of the two finalists: The 28-year-old Federer’s guest box was full, with pals such as rock-star couple Gwen Stefani and Gavin Rossdale and Vogue editor Anna Wintour seated alongside Federer’s parents, wife and agent. Only three of the 15 available seats were occupied in del Potro’s box.

Federer took a 3-0 lead in 15 minutes, winning one point by racing about 5 feet wide of the doubles alley for a defensive backhand, then sprinting the other way for a cross-court forehand passing winner that he celebrated by yelling and shaking his fists.

He even took time to watch a replay on a stadium video screen. Not quite the “Did he really just do that?!” sort of trick shot Federer pulled off against Novak Djokovic in the semifinals — a back-to-the-net, between-the-legs, cross-court passing winner to get to match point — but pretty spectacular, nonetheless.

But del Potro eventually got going, swinging more freely and taking full advantage of Federer’s serving woes: 11 double-faults and a first-serve percentage of only 50.

Used to traveling without a full-time coach, Federer generally is quite adept at making mid-match adjustments and dealing with opponents’ switches in strategy. But it was del Potro who realized he needed to put full belief in the strength of his 100 mph forehands and not worry about too much else.

That tactic worked, and Federer never found a way to counter it, losing leads in the second set and the fourth set. He was up 5-4 in the fourth, and at 15-30 on del Potro’s serve, Federer needed only two more points to become the first man since Bill Tilden in 1920-25 to win the American Grand Slam tournament six years in a row.

Del Potro held steady there, and Federer would never come that close again.

While hardly a household name, del Potro was not an unknown in the tennis world. He burst onto the scene a year ago with a 23-match winning streak and four tournament titles in a row on hard courts, the surface used at Flushing Meadows. There also was a bit of a harbinger for this back when del Potro presented problems for Federer in the French Open semifinals in June, taking a 2-1 lead in sets before frittering that away.

Federer went on to win the title at Roland Garros, his first there, to complete a career Grand Slam and tie Pete Sampras’ career record of 14 major championships. Federer then broke that mark by collecting No. 15 at Wimbledon.

Thanks to del Potro, Federer will have to wait for No. 16.

From mid-May until Monday, Federer had been 32-1 with four titles from five tournaments. Aside from the on-court success, Federer’s 2009 included getting married and becoming a father — of twins, no less.

Quite a year. Still, one can’t help but ponder this: No man has won even three straight major tournaments in a season — much less all four — since Rod Laver’s true Grand Slam in 1969. Federer came close this year, his French Open and Wimbledon titles bookended by a five-set loss to Nadal in the Australian Open final and a five-set loss to del Potro in the U.S. Open final.

This U.S. Open was Federer’s first Grand Slam event since his daughters were born, and he spoke proudly of quickly learning to change diapers and getting used to sleeping less.

“Right now, I’m just tired,” he said after his loss. “I want to get a rest.”