NEW YORK. Sept 8 (AP/Reuters): Pete Sampras strolled off the court after overpowering yet another young challenger. And an apparent runaway victory turned Into a dramatic triumph of heart and courage for fifth seed Venus Williams as she held off the challenge of France s Mary Pierce 6-1. 7-6 to earn a berth in the quarterfinals.

On a Monday when thunderstorms turned the US Open into Wimbledon West. Sampras reached the quarterfinals with a convincing win over a dangerous opponent.

But Andre Agassi was struggling to avoid elimination In a match against Karol Kucera in which Agassi mocked his opponent and the players postured at each other across the net. Kucera was up two sets to one when rain halted play for the night.

Agassi, who had complained that Kucera was stalling by practising his service toss before hitting serves, began mimicking Kucera's practice tosses. The two then exchanged moon-balls. lofting soft shots back and forth.

Kucera led 6-3. 6-3. 6-7. 0-3 when play was stopped. It was the second rain delay of the match, which will resume Tuesday afternoon.

Sampras had 18 aces and never lost his serve in a 6-4. 6-3, 6-2 win that included a 2 1/4-hour rain break, turning his power lunch with Marat Safin into a late-aftemoon snack for the world’s top player.

"I was craclcing it pretty hard throughout the match." Sam peas said. "I got the serve going, which has been a little bit up and down throughout this past week. Today it seemed like it clicked."

The top-seeded Sampras, a four-time US Open champion, next will face the winner of the match between No 8 Agassi and No 9 Kucera.

Sampras lost only 20 points on his serve In the maten and closed out his 600th career win with three consecutive aces — including one at 135 mph (216 kph). He won 41 of 48 points on his first serve in the match.

Safin. 18. who wowed crowds at this year's French Open with a string of upsets en route to the fourth round In his Grand Slam debut, showed off plenty of power, including 10 aces. But he was no match for the steady Sampras.

"He's very talented, but also very young. The first set I couldn’t believe the pace of his serve. I really had a hard time reading it. Trie talent is there, maybe he just needs to tone it down a little bit.' Sampras said. He'4 IB years old and he's got, plenty of years to learn from mistakes. My game at 18 was pretty bad."

Agassi was pumped up. With his wife, actress Brooks Shields, cheering wildly, he

held for 5-5. pumping his fist at Kucera and yelling Come ont"

The eighth-seeded American then ran off eight straight points to win the 12th game and take a 4-0 lead In the tiebreaker

before converting his first set point of the rainy evening on a blistering forehand into* the corner.

The momentum held through the opening three games of the fourth set. before a light mist turned into steady rain.

Williams was facing a likely third set trailing 4-1 In the tiebreaker before she rattled off the last six points to claim the breaker 7-4 for a hard-fought one hour. 35 minute win.

But Williams could not wrap up the match, dropping her serve at 30-40 for the first time in the match to set up the climactic tie-break.

Earlier, South African Amanda Coetzer beat longtime nemesis Conchita Martinez, the seventh seed. 4-6. 6-4. 6-2 to earn a spot in the quarterfinals for the third time in the last five years.

"It’s very nice. Definitely she's been a very tough opponent for me my whole career." said the 13th-seeded Coetzer. who had won one of 13 previous meetings with the Spaniard.

"It's great, especially because it's at a Grand Slam."

Martinez was pained by her 57 unforced errors and a sore neck throughout the two hour. 38 minute match.

"It was difficult for me to play my own game because I was injured," said Martinez, who had a trainer work on her neck several times during changeovers. "

"I thought of' retiring since the first set. but I thought, hang in there, hang in there."

Coetzer will now play second seeded Lindsay Davenport in the quarters. The third ranked

American defeated 10th seeded Wimbledon runner-up Nathalie Tauziat 6-1.6-4.

Williams quarterfinal opponent will be No 4 Spaniard Arantxa Sanchez-Vicario, who won 7-6, 6-3 over No 15 Anna Kournikova of Russia.