The Armenian Express finally ran out of gas yesterday, as a spent Sargis Sargsian could barely muster enough energy to make his friend and former workout partner Andre Agassi break a sweat over three easy sets.

Sargsian, an unseeded veteran and fast favorite with fans, had been on the court for more than 12 hours before his match yesterday. He defeated Olympic Gold medalist Nicolas Massu in five hours and nine minutes, the second longest match in U.S. Open history, and followed it up by coming back from two sets down against Paul-Henri Mathieu to win in four hours and 41 minutes.

Yesterday, though, it was clear from the outset that the 31-year-old Sargsian did not have the weapons to hurt Agassi, 34, who now has won all six of their meetings. The Armenian lasted only an hour and 30 minutes in losing 6-3, 6-2, 6-2, and he said afterwards that the trademarks of his game, consistency and hustle, do not match up well against Agassi’s precision and ability to dictate points.

“I don’t feel like I have a game plan against him,” Sargsian said. “You cannot serve and volley, you cannot play him from the back, you cannot hustle because you know he’s not going to miss, he’s going to make you run till tomorrow morning. I don’t like playing him. He just plays so fast. He rushes you so much. It’s very uncomfortable.”

For Sargsian, 2004 had been a disappointing season until he arrived at the Open. He’s never been a force on the tour, but he won praise from Agassi yesterday for his athleticism and toughness. Sargsian came to the United States from Armenian at age 20 before finding an adoptive family and spending two years honing his game at Arizona State University, where he became the top college player in the country and won the NCAA singles championship in 1995 as a junior. He matched his best ever Grand Slam performance by reaching the round of 16 here.

“It’s been amazing,” Sargsian said of his run. “I didn’t feel like I was playing my best coming to this stretch. To pull out matches like I pulled out against Massu and Mathieu, I don’t know, it’s unbelievable.”

Agassi’s victory set up the most anticipated match of this year’s tournament: Agassi, the aging legend, versus top-seed and world no. 1 Roger Federer in the quarterfinals, Wednesday evening under the lights at Arthur Ashe Stadium. Federer was scheduled to play last night, but his opponent, Romania’s Andrei Pavel, announced during the afternoon that he would be forced to forfeit due to a herniated disc in his back. Pavel, 30, said rest ought to fix the problem, but he added, “I’d rather quit playing tennis than have surgery on my back.”

After his walkover, Federer will take the court against Agassi without having played a match since Saturday. Agassi, at least, hoped that the extended layoff might make Federer a little rusty around the edges.

“I think not playing in four days is not ideal for him,” Agassi said.

Federer said the unexpected forfeiture jostled his routine a bit yesterday, but he said he would get over it by tomorrow. “Practice was not horrible, but not enjoyable because that’s not what I came here for,” he said. “But then, you know, when I go to bed tonight, I’m happy I’m in the quarterfinals.”

Agassi and Federer have played exceptional matches in the past, winning three each. Federer, however, has won the last three, including a two-sets-to-one victory this year and two victories at last year’s Tennis Masters Cup in Houston.

Earlier in the day, Britain’s Tim Henman and Germany’s Nicolas Kiefer were engaged in a tense, five-set match when Kiefer suddenly bowed out with a wrist injury trailing 6-7(5), 6-3, 6-1, 6-7(4), 3-0. Kiefer, who was serving, said he hurt himself when he hit a reflex backhand volley off a hard struck ball by Henman.

“I felt that something happened,” he said. “I just tried to keep on going. But I couldn’t hold my racket.”

Kiefer tried to hit another serve and then called for the trainer. Per Bastholt, who examined Kiefer, said the ailment seemed serious and was probably a problem with a tendon. “He had absolutely no power,” Bastholt said. “He wouldn’t want to extend the wrist.” An MRI showed a light tendon tear that could keep Kiefer out of action for three to six weeks.

Kiefer’s withdrawal was an unfitting end to a close contest that saw Henman, celebrating his 30th birthday, mired in his third fifth-set match of the tournament. He fell behind early in the first set, and then came back to tie it at six games all, only to lose the tiebreak. After that, Henman seemed to have Kiefer dead to rights. He rolled over him for the next two sets, as Kiefer made a few horrible errors on short balls and a few easy volleys. Kiefer, however, pulled himself together and won the fourth set before falling behind in the fifth.

Henman’s opponent in the quarterfinals will be Slovakia’s Dominik Hrbaty, who ended a magical run by Belgium’s Olivier Rochus, the smallest player in the tournament at five feet five inches and 130 pounds. Hrbaty came back from two sets down to win, 2-6, 3-6, 6-3, 6-4, 6-0 in three hours. Rochus, who had two previous matches reach five sets, suffered from cramps as the match progressed and could not stay with Hrbaty.