NEW YORK — 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 yesterday.

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.

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

"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."

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.

SERENA WINS DOUBLES WITH SISTER; APOLOGIZES

After two days to think about it, Serena Williams wishes she could give the line judge she yelled at "a big ol' hug."

Williams teamed with sister Venus yesterday to win the U.S. Open doubles title with a 6-2, 6-2 victory over Cara Black and Liezel Huber.

After being called for a foot fault toward the end of her match Saturday against Kim Clijsters, Williams directed a profanity-laced tirade at the line judge that led to a point penalty that ended the match.

Williams was fined $10,000 Sunday.

Asked what she'd do if she saw the line judge again, Williams replied: "I'd like to give her a big ol' hug and put it all behind us like I have. Move on from it like I have and just learn."