MELBOURNE, Australia (AP) -- One Williams down, one to go for Martina Hingis.

The top-seeded Hingis spoiled a potential sibling showdown at the Australian Open by winning a dramatic marathon from Serena Williams 6-2, 3-6, 8-6 to complete the women's quarterfinals Wednesday.

Williams' sister, Venus, earlier overcame a slow start and a late deficit to edge Amanda Coetzer in another extended third set, 2-6, 6-1, 8-6.

In Thursday's semifinals, Hingis will renew her rivalry with the third-seeded Venus Williams, who needs two more victories for her third consecutive Grand Slam tournament title. Hingis leads the matchup 9-7, but Williams won both meetings last year en route to titles at Wimbledon and the U.S. Open.

Defending champion Lindsay Davenport, seeded second, will play No. 12 Jennifer Capriati in the other semifinal.

Hingis is bidding for her sixth Grand Slam tournament title but her first in two years. She trailed 4-1 in the third set and was two points from defeat at 5-4, deuce. But she broke Williams' serve to reach 5-5, then broke again in the final game, winning the second match point with an overhead smash.

Both players played their best tennis in the final few games.

``It was a great comeback,'' Hingis said. ``I was just trying to hang in there. I wasn't feeling that great.''

Neither was Williams, who complained of an upset stomach. She received treatment from a trainer during several changeovers, and she sometimes grimaced or bent over in apparent discomfort between points.

Williams also struggled with her serve and committed 54 errors. But Hingis needed 2 hours, 19 minutes to secure the victory.

``It was a great effort by both of us, and I was lucky,'' Hingis said.

Three times Hingis has defeated one Williams sister and then lost to the other in the same event, most recently at the 1999 U.S. Open, when Serena beat her in the final.

Only twice has a player beaten both sisters in the same tournament: Arantxa Sanchez-Vicario at Sydney in 1998 and Monica Seles at Sydney in 1999.

It was almost a Coetzer-Hingis semifinal. Venus Williams trailed Coetzer 5-3 in the final set and won despite a whopping 56 unforced errors.

``I pretty much fought for everything I got,'' Venus Williams said. ``Toward the end I cut down on my errors and moved forward. I had been in that situation many times before. I had the experience of being in the hole.''

Defending men's champ Andre Agassi, seeded sixth, will play No. 12 Patrick Rafter in the men's semifinals Thursday night. Rafter reached the final four in his country's biggest tournament for the first time by beating No. 14 Dominik Hrbaty 6-2, 6-7 (4), 7-5, 6-0 Tuesday night.

No. 5 Yevgeny Kafelnikov was the top-seeded player remaining in the other half of the draw.

Venus Williams is 11 inches taller than the 5-foot-2 Coetzer and hits her serves 25 mph faster, which made their pairing look like a mismatch. For a while it was.

Coetzer, seeded 10th, was content to keep the ball in play and wait for errors by an erratic Williams, who lost the first eight points, lost 16 of the first 19 and fell behind 4-0. In the first set she committed 19 errors and hit just one winner.

``Let's go, Venus, wake up!'' a fan shouted.

She did, holding serve with an ace to start the second set, then breaking for the first time when Coetzer double-faulted. Mistakes began to creep into the South African's game, while Williams tamed her wild strokes somewhat.

``I really had to pick myself up if I wanted to stay in the tournament,'' Williams said. ``I don't like to lose, and I just really didn't want to.''

But the rallies became especially sloppy down the stretch. Coetzer served for the victory at 5-3 but quickly committed four unforced errors and lost the game. Williams then had a chance to serve it out at 6-5 but won only one point, double-faulted and hit three groundstrokes wide to lose the game.

Williams broke again, then concluded the ragged victory with an overhead winner. She smiled and sagged her shoulders in relief at her second consecutive three-set win.

``Today,'' she admitted, ``was not one of my better days.''

The start of the match was delayed half an hour by a shower that caught tournament officials by surprise. Play began after the retractable center court roof was closed for only the second time in the tournament.

Asked about playing indoors, Williams said she regretted wearing a cap during the match.

``It affected my hairstyle,'' she said. ``If I had known they were going to close the roof, I would have worn a ponytail.''