MELBOURNE, Australia (AP) -- In the burning heat or under the roof during rainstorms, Martina Hingis rolls on unperturbed by the elements or her opponents in the Australian Open.

Hingis, the three-time defending champion, won her 24th straight match in the Australian Open on Saturday, repelling the fierce serves of rising star Alicia Molik to advance to the fourth round 6-2, 6-3.

In seeking to become the first woman in the open era to win four consecutive Australian Open titles, Hingis has lost only 14 games in three matches.

``Three in a row younger than me -- I feel like I'm getting into middle age,'' the 19-year-old Hingis said.

Against Molik, an 18-year-old Australian who served at up to 116 mph, Hingis looked absolutely unruffled, as if she could stand there and swat back returns all day.

With the retractable roof closed because of rain midway through the first set, Molik did serve six aces. Hingis accepted those occasional balls flying past as inevitable, shrugged them off, and waited for others to take aim at. On her own serve, Hingis didn't try to match Molik's pace but nevertheless gave up only five points.

``Her serve was just big,'' Hingis said. ``Sometimes I would think, `OK, there's nothing I can do about it.' I knew I wasn't allowed to lose a rally.''

Molik tried rushing the net on her second serves, with mixed success, but was helpless when Hingis kept her back and rushed to the net herself.

``Martina has the best return of serve in the game,'' Molik said. ``You know that quality return is coming back at you, and it puts you under more pressure, so you push that little bit harder and it throws you off your game.''

In the only matches completed amid intermittent rain, the men's No. 4 seed Nicolas Kiefer beat Karim Alami 6-3, 6-4, 6-2; and Arnaud Clement downed Roger Federer 6-1, 6-4, 6-3.

Andre Agassi doesn't dwell on regrets, on or off the court, though there is one piece of advice he wishes he could tell his younger self: Cut those long, frosted locks.

``Every time I see a picture of me with long hair, I want to burn it,'' Agassi said.

Approaching 30 in a few months, Agassi's shaved scalp is symbolic of the simplicity he is seeking in his tennis game, a single-mindedness on drawing as much as he can from the time he has left.

To that end, Agassi came to the Australian Open in perhaps the best shape of his life, his body leaner, more muscled and snappingly quick after a winter of workouts than it was during last summer's scintillating run from the French Open to the U.S. Open.

His strength and speed were on display Friday night in a 6-4, 6-4, 6-2 victory over Argentina's Mariano Zabaleta, a match that to Agassi's relief provided none of the tension of Pete Sampras' earlier comeback from two sets down against Zimbabwe's Wayne Black.

Seeing Sampras struggle against a qualifier might have heightened Agassi's sense that danger lurks in every match.

``It gets your blood going a little,'' Agassi said. ``I didn't want to see that upset today because we're going to be playing Zimbabwe in 10 days' time. But it just goes to show how strong the field is here.''

If Agassi got anything out of his match, it was the knowledge that he could repel Zabaleta's 130 mph serves on the Australian's souped-up court with the roof closed. It was important because in the next match Agassi faces the fastest server in tennis, Mark Philippoussis.

``I expect I'm going to have to do some special things on Sunday,'' Agassi said.

``He's only going to give you a few looks at breaking him, and you've got to make sure that when you break him you're not breaking him to get back into the match. You've got to take care of your serve, and you've got to be ready to fight off everything and keep the pressure on him. He likes to dictate the play and I don't blame him.''

Agassi is no less quick in his reflexes on returns as he was when he turned pro in 1986 or when he reached his first major finals at Paris and New York in 1990. He was the image-is-everything kid then, the rock-star-as-tennis-player, and he came to realize years later that the image didn't suit him.

These days, Agassi looks pared down, top to bottom, like a swimmer. He eschews the old baggy shorts and floppy shirts look in favor of tighter outfits that emphasize his slimmer profile. He likes to show off his physique in practice by going bare-chested or wearing muscle shirts with the sleeves cut off at the shoulders.

Against the No. 16 Philippoussis, the top-seeded Agassi will face an opponent who is five inches taller at 6-foot-4 and 30 pounds heavier at 200. But tennis is not boxing, and Agassi will go into the match favored by aficionados if not by the fans in Philippoussis' hometown.