NEW YORK -- It would be difficult to overstate Roger Federer's reign of dominance here at the Billie Jean King National Tennis Center.
Coming into his championship final match with Juan Martin del Potro, he had won 40 consecutive matches and was looking for his sixth consecutive title, something that hadn't been achieved in 84 years. In successive finals, he had beaten, in order, Lleyton Hewitt, Andre Agassi, Andy Roddick, Novak Djokovic and Andy Murray.
On Monday, Federer ran into a fearless 20-year-old from Argentina who, after a first set marred by nerves, was not awed by the aura Federer has worked so hard to craft. This was del Potro's first Grand Slam final. The Swiss star, acknowledged widely as the greatest player ever, was appearing in his 17th of the past 18.
Del Potro, an underdog of almost unimaginable proportion, rode his massive forehand to a 3-6, 7-6 (5), 4-6, 7-6 (4), 6-2 victory for his first major title.
After his 6-2, 6-2, 6-2 semifinal victory over Rafael Nadal, del Potro called it the greatest moment of his life. How did this feel?
"Much better," del Potro deadpanned in his on-court interview.
The last match of the Grand Slam season ran 4 hours, 6 minutes -- the longest final here in 20 years -- and it offered extraordinary quality along with quantity. Federer, almost always a lock in major championship tiebreakers (he was previously 18-3), uncharacteristically lost both extra sessions to del Potro.
"It's one of those finals, maybe I'll look back and have some regret," Federer said. "In the end, he was just too tough. That's the way it was.
"His effort was fantastic."
Del Potro can't legally celebrate with a bottle of Dom Perignon, but he is the first player to beat Nadal and Federer in a Grand Slam tournament. At 6-foot-6, he also becomes the tallest Grand Slam champion in the Open era.
"It's amazing for me," Del Potro said. "I'm very happy to be with this crowd, this people, this court. This will be in my mind forever in my life. I don't have words for this."
The last player to win his first Grand Slam title at the U.S. Open was Andy Roddick in 2003.
"I had a great run myself, but he was the best," a gracious Federer conceded. "Of course, I would have loved to win. I never would have believed I could win 40 in a row here. It's been an amazing run for me."
Del Potro overcame his unsteady start and, after serving to stay in the match in the third set, two points from defeat, played with astonishing confidence in the late going. In the fourth set, after he sprinted to track down a sharply angled shot from Federer, following with a nifty little forehand winner, del Potro nearly ran into the first row at Arthur Ashe Stadium. He high-fived a dozen fans before he returned to the baseline.
Federer, meanwhile, slowly lost his composure.
He was infamous for his temper as a junior, but in putting together the greatest sustained streak of excellence in the game's history, Federer learned to master his emotions. On this night, he dropped several choice and inappropriate rejoinders on chair umpire Jake Garner that were worthy of Serena Williams. He questioned line calls and let the irritation linger. He grew agitated when Garner declined to play a point over after a fan yelled "out."
Federer, in the end, looked disheveled. His forehand unraveled;, he didn't serve well (low tosses dogged him all night); and when opportunities presented themselves, in the form of break points, he did not take them. Federer had 22 chances and converted only five.
Going in, the history -- almost all of it -- was against del Potro.
Federer had won each of their six previous matches. The only encouraging thing? After losing the first 12 sets to Federer, del Potro won two of three in their semifinal encounter earlier this year at Roland Garros before losing in five.
In his semifinal win over Rafa, del Potro fairly oozed with confidence. Early in Monday's final, that swagger -- and his cranked-up forehand -- were missing in action.
The second game of the match set a tone that ultimately proved to be irreversible. With a nervous del Potro serving, Federer pressed him continually and scored five break points; the fifth was the charm. Federer won a crazy, scrambling point -- the best of the match -- making two terrific gets and finishing it with a savage forehand crosscourt winner.
That elicited an uncharacteristically early fist pump from Federer and dropped del Potro into what seemed to be a deep hole.
Sixty-nine minutes later, it was del Potro who was launching a rousing uppercut fist pump of his own. Federer had been serving for the set at 5-4 when del Potro, for the first time, started playing as if he thought he could win.
At 30-all, del Potro hit a forehand down the line that was called out, but a challenge replay revealed it had nicked the line. On break point, del Potro hit a gorgeous running forehand loaded with topspin and sidespin that hooked into the corner for 5-all. The tiebreaker went to the Argentine when he converted his third set point with a crosscourt forehand winner.
"It's a pity," Federer said. "If I win that second set, I'm in a great position to come through."
The third set was similarly contentious. When del Potro considered a challenge, while Federer was on his way to his changeover chair with a 5-4 lead, Federer assailed Garner, who told him to be quiet. Abusive language ensued.
Del Potro, unstrung by a netcord that went against him, double-faulted twice to give agitated Federer the third and seemingly pivotal set. This is when del Potro grew more sure of himself and Federer, at the same time, began to let niggling things distract him from his greater purpose.
"At that moment I start to think I'm playing with Roger, the best player of the history," del Potro said, "Nothing to lose. Keep fighting. I started to believe in my game. It helped me."
The fourth-set tiebreaker wasn't really close; Federer's forehand couldn't find the court. The fifth set was, startlingly, a blowout. On del Potro's third match point, Federer ripped a backhand long and the young Argentine fell back on the court and sobbed.
"Juan Martin played great," Federer said. "He hung in there and played great in the end, he was a better man. I had chances today to win, but couldn't take them. It was unfortunate."
Nine years ago, a swashbuckling 20-year-old Russian lit up Pete Sampras and the U.S. Open. Marat Safin won his first Grand Slam title, and these del Potro wins over Nadal and Federer, the game's finest, had that sort of feel and texture.
"I have to compare it to my first at Wimbledon," Federer said. "It's the best feeling on the planet. It's kind of unexpected. It's good to see him so happy and emotional about it.
"He should enjoy it. He deserves it.
Federer mused aloud about his rivalry with Nadal and added, "Who knows, maybe del Potro's going to join that thing in the future."
Less than an hour after he won, del Potro seemed subdued. Maybe it's because he didn't sleep much the night before the match.
"When I laid down on the floor, many thing came to my mind," del Potro said. "Family and friends. It's my dream, it's done. I will go home with the trophy. It's the best sensation of my life.
"Maybe next week I will believe in this. I don't understand nothing."
Greg Garber is a senior writer for ESPN.com.