He rode into the tournament on a wave of momentum and popularity after his performance in the Wimbledon final, and for some, Andy Roddick was in position to ride that all the way to his second United States Open title.

But the wave crashed too soon, and Roddick’s resurgent summer ended under the lights in Arthur Ashe Stadium before the second week of this season-ending tournament even began.

John Isner, the 6-foot-9 slammer from the University of Georgia, upset Roddick, the world’s No. 5 ranked player, 7-6 (3), 6-3, 3-6, 5-7, 7-6 (5), Saturday in 3 hours 51 minutes.

In a stirring confrontation between two of the biggest servers in the game, Isner won the first two sets, lost the next two, then overcame that disappointment and cramps in his right leg to reach the fourth round of a major for the first time.

In the fifth-set tiebreaker Isner took command at the net, putting away a backhand volley at 5-3 to get to a match point. Roddick came back with two consecutive aces to make it 6-5 and the serve went back to Isner. Roddick returned well, but Isner came to the net and Roddick could not pass against Isner’s long wingspan, hitting a running forehand into the net.

After Roddick saved three match points, Isner finally broke through. At the moment of victory Isner fell to the ground, but quickly got back up and received a warm hug from Roddick.

For Roddick, who had lost weight and gained confidence this year after adding new coach Larry Stefanki, it is back to the practice courts and the indoor season before he will have another chance at a major tournament title next year in Australia.

The 2003 Open champion, Roddick pushed Roger Federer in five emotional sets before losing in the final at Wimbledon in July, gaining popularity after that loss and also seemed to have straightened out his game and redirected his career toward this tournament.

The challenge here was never going to be easy, though, not with Novak Djokovic and Federer on his side of the draw. But few could have anticipated that, instead of one of those top players ousting him, it would be Isner, the 55th ranked player in the world, who had never even broken Roddick’s serve in two previous meetings.

Isner is known for his blistering serve and his small forward’s physique, but on Saturday he also showed some nerve and some deft touch, refusing to succumb to the more experienced Roddick who was playing on what has become his home-court advantage — under the lights on Ashe.

Isner had only played in five previous Grand Slam events, whereas Roddick had played in five major finals, including two at the Open, one of which he won. And no player has been on center court at night more than Roddick.

The match was the last scheduled during the day session, and Isner won the first two sets in daylight. The key moment of the first set came in the eighth game when Roddick had three consecutive break points against Isner.

But Isner reeled off 5 consecutive points to even the set at 4-4, and he would go on to win the tiebreaker with relative ease at 7-3.

In the second set Isner, who came into the match with an 0-2 record against Roddick (both losses came on hard courts in Washington) finally broke him. He played even more aggressively than he did in the first set, scoring 16 winners, to only 5 by Roddick, and closed out the set with his 17th ace.

But when darkness fell and the lights came on, Roddick surged to life. Once again he earned three consecutive break points on Isner’s serve at 3-4, but this time he was able to convert.

After two aces from Isner — who had 38 for the match — made it 30-40, Isner hit a short volley and Roddick, running sideways, hit a deep backhand down the line that grazed the baseline. His momentum carried him to the retaining wall near the front row of seats and Roddick just kept pumping his fist. He then served out the set with the exclamation of an ace.

Roddick had only come from behind twice to win a five-set match after losing the first two sets, at the 2003 Australian Open against Mikhail Youzhny, and at the 2003 United States Open against David Nalbandian.

Isner’s performance was all the more impressive because he spent three months, from mid-April to mid-July, recovering from mononucleosis. But he played well in his comeback, reaching he semifinals at Indianapolis and Washington, and the quarterfinals at Los Angeles.

Before the match Roddick noted the progress Isner had made this year.

“I think he got a little bit too passive in his game from there,” Roddick said. “Now he’s been working with Craig Boynton for a little while. I think Craig has made a huge difference. Especially coming off being sick, he’s certainly made some good strides this summer.”