MELBOURNE, Australia - Martina Hingis found out she still can slug it out with the best of 'em. She also knows she has a lot of work to do.

Second-seeded Kim Clijsters overcame a rash of mistakes and ended Hingis' impressive comeback run at the Australian Open, winning 6-3, 2-6, 6-4 today to reach the semifinals and take over the No. 1 ranking.

“It's been a long time since I played her,” Clijsters said after Hingis, a three-time champion here, exited Rod Laver Arena to a loud ovation. “She's improved a lot since she was at her best.”

But that was when the power surge in women's tennis was just picking up momentum. While Hingis showed that she wasn't overmatched from the baseline, her weak serve let her down, setting up easy winners for Clijsters.

Asked if she learned anything, Hingis said: “Speed is important.”

Despite the loss, Hingis was satisfied with her first Grand Slam event in three years.

“I think I can be proud of myself,” Hingis said. “You just can't think you're going to go out there and win everything. Losing to Kim today still gives me so much to look forward to. I just have to work harder. I have to start believing.”

The victory guaranteed that reigning U.S. Open champion Clijsters will take over the top ranking from Lindsay Davenport, who lost in the quarterfinals.

“After the year I had last year, this is the cherry on the cake,” she said. “It was never a goal. Keep working hard and good things happen.”

She next plays No. 3 Amelie Mauresmo, who reeled off the last nine straight games in a 6-3, 6-0 quarterfinal win over No. 7 Patty Schnyder.

On the men's side, top-ranked Roger Federer struggled for the second consecutive match but still advanced to the semifinals by ousting No. 5 Nikolay Davydenko of Russia 6-4, 3-6, 7-6 (7), 7-6 (5), fending off six set points in the third set before taking it in a tiebreaker.

“I'm extremely, extremely happy to come through tonight,” Federer said. “I really had to fight. It was hard.”

Federer will play No. 21 Nicolas Kiefer of Germany, who survived a nearly five-hour match - and a 96-minute final set - to defeat No. 25 Sebastien Grosjean of France 6-3, 0-6, 6-4, 6-7 (1), 8-6.

Hingis quit the tour in 2002 due to persistent foot and heel injuries. Ranked No. 349 and playing on a wild-card entry, she returned to competitive tennis on Jan. 2 and was 7-2 coming into the match.

But the best player that she had beaten here was 30th-seeded Vera Zvonareva in the first round, and everyone was eagerly waiting to see what would happen when she faced a top player like Clijsters.

Hingis looked nervous, tentative and overmatched at the start, as Clijsters won 12 of the first 16 points and jumped ahead 4-0.

With shadows creeping across the court, the crowd began almost evenly split for the two players but shifted quickly to the beleaguered Hingis, who twice angrily whacked balls into the net after missing easy volleys.

Clijsters was picking up easy points off her serve and pouncing on Hingis' weak serves, which often were 20 mph slower.

Although she faltered early and often found herself lunging for stinging groundstrokes deep in the corners, Hingis stayed in the match with her defense.

Clijsters led by a set and a break, and an easy victory seemed certain. Then, Hingis settled in and started measuring Clijsters' speed as the Belgian made numerous errors.

Clijsters, who has been suffering from hip and back pain, was looking stiff, spraying shots all over the place, and finished with 48 unforced errors. But she pulled herself back together in the third set.

“I was feeling very weird out there today,” Clijsters said. “I felt very empty. The second set wasn't good at all. I just hit the wall. All of a sudden, I just felt no power in my legs to push off.”

Hingis said the difference was partly in her head.

“I always used to have this mental edge over other players,” she said.

The comeback will continue, with Hingis heading next to Tokyo while mapping out which tournaments - and how many - she will play in this year.

“You think I'm going to give up right now?” she asked.

Kiefer already had been fined for making “audible and visible obscenities” in his first-round, five-set win over Paradorn Srichaphan of Thailand. Today, he frequently questioned line calls and lost his cool as he dropped the fourth-set tiebreaker, and again when he was broken for a second time in the fifth set, getting warnings for obscene language. One more code violation would have cost him a point.

In a bizarre point at 40-30 in the 12th game of the fifth set, Kiefer tossed his racket over the net just after Grosjean - serving to stay in the match - hit a forehand into the net.

Grosjean immediately appealed for a hindrance ruling, but was denied by umpire Carlos Bernardes and then argued unsuccessfully with Grand Slam supervisor Mike Morrissey.

When the point was confirmed for Kiefer, making it deuce, the crowd erupted with loud boos and whistling.

In the end, Grosjean held the game, but was broken the next time he served at 6-7 to finish the match in 4:48.