By Steven Wine
The Associated Press

KEY BISCAYNE, Fla. - For more than an hour Andre Agassi found himself on the defensive, chasing after shots in vain and watching winners whiz by, just trying to hang on until Mark Philippoussis cooled off. It finally happened early in the third set.

The big-hitting Philippoussis dumped a double fault into the net on break point, and Agassi took the lead to stay, winning 4-6, 6-3, 6-2 in the fourth round of the Nasdaq-100 Open on Tuesday night.

"There's no doubt he's more mentally tough than me, obviously," Philippoussis said.

Agassi converted only four of 14 break-point chances, but he remained patient until the Australian's game began to unravel. The pivotal double fault gave Agassi a 2-1 lead in the final set, and a flurry of errors by his opponent followed.

Philippoussis finished with 28 winners but also 38 unforced errors.

"He's an aggressive player, no question," Agassi said. "He takes the match in his own hands on so many shots. So you always want to leave room for the possibility of him just missing. That's as good of a win as any, if you can get your opponent to miss."

Seeking his third straight Key Biscayne title and his sixth overall, the No. 2-seeded Agassi will play No. 19 Younes El Aynaoui in the quarterfinals Thursday. The 32-year-old Agassi will be joined in the final eight by two other Americans: 32-year-old Todd Martin and 20-year-old Robby Ginepri.

Ginepri gave the resurgence in U.S. men's tennis another nudge by defeating Hyung-Taik Lee 6-3, 6-2. The victory made Ginepri the only man to reach the quarterfinals at both Indian Wells earlier this month and Key Biscayne.

The quiet Georgian is assured of breaking into the top 50 next week, and he'll bid for the first semifinal berth of his career Wednesday against former No. 1 Carlos Moya.

"It's all coming together right now," said Ginepri, who turned pro two years ago. "The experience has kind of sunk in now, and I know what to expect. Just being out here with all the top players, day after day, I'm finally getting used to it."

Martin, who arrived at Key Biscayne with just one match victory this year, eliminated Radek Stepanek 6-3, 6-2.

Moya, seeded fifth, erased four set points in the second set and beat Nicolas Escude 6-4, 7-6 (8). Fourth-seeded Roger Federer defeated No. 14 Sjeng Schalken 6-3, 6-2. Other winners were No. 9 Albert Costa and No. 13 Paradorn Srichaphan.

Top-seeded defending champion Serena Williams and No. 3 Kim Clijsters easily advanced to a showdown in the women's semifinals Thursday. Williams lost her serve three times but still beat French qualifier Marion Bartoli 6-1, 6-2. Clijsters routed No. 9 Jelena Dokic 6-2, 6-0.

Hopes for an all-Williams final were dashed when Venus lost to Meghann Shaughnessy on Monday night.

"I'm really upset and disappointed," Serena said. "At least it's not Wimbledon."

Agassi skipped Indian Wells earlier this month because of a sore right shoulder, but he said the injury wasn't a factor in his conservative approach against Philippoussis. He hit a lot of first serves at 90 mph but also smacked some in excess of 115.

"I'm always aware of the shoulder, just because I'm always worried about it," he said. "There's a little bit of a general fatigue, but nothing that prevents me from being able to muscle through it."

He improved to 15-1 this year and beat Philippoussis for the sixth straight time. For the Aussie, the latest defeat was especially galling because he had Agassi on the ropes for much of the match.

"I wanted to make him play, make him earn it," Philippoussis said. "I sort of felt like I gave it away again, and that's what's frustrating."

Agassi extended his winning streak at Key Biscayne to 15 consecutive matches. The latest victory could be a good omen for him, because Philippoussis has lost to the eventual champion at three tournaments already this year.