America's Venus Williams toppled Martina Hingis in the Sydney International tournament in a thrilling second round match where the lanky teenager found some chinks in the armour of the world number one.

It was a stunning day for the flamboyant Williams sisters, with 16-year-old Serena defeating up-and-comer Mirjana Lucic by the same margin as her elder sister despatched Hingis - 3-6 6-4 7-5.

"We want to be at the top together," Serena said. "If I play my game and she plays her game, then we'll both be fighting for those (one and two rank) spots."

The elder Williams said the dramatic, three-set match was far different from their encounter at last year's US Open finals where she crumbled under the world number one.

"She was more vulnerable this time," said the supremely confident Williams, who had failed to take a set of Hingis in three previous matches.

"A lot of girls are stronger than her and some are faster than her," she said, tossing her distinctive, beaded locks.

The towering 17-year-old battled cramps in the sweltering conditions but was too strong for Hingis, who appeared unfit and irritable.

After some scrappy play in the first two sets, Williams was troubled by cramps in the sweltering conditions, but a teary spell on the sidelines while a physiotherapist worked on her appeared to focus her game.

"I didn't want to quit, I had come so far in the heat, I knew I couldn't give up," she said.

Hingis took the loss in her stride, admitting she was unfit and had played passively in the final set, waiting for Williams to continue making the errors that had littered her earlier play.

"But it's not disappointing because now I have something to learn from . . . especially for Melbourne," said the ever-positive Hingis who will now depart for the Australian Open.

"She had nothing to lose . . . it's not as important as the US Open," she said defiantly, referring to last year's Grand Slam triumph.

"I should have won this match but she played better, played smarter."

Hingis acknowledged that her ranking could be captured by one of the sparky new players on the circuit, including Russia's Anna Kournikova and the Williams sisters - all of whom are playing here this week.

"It's going to be a great year in that so many new players are coming up - and you can't forgot the old ones," she said.

Williams will play Magdalena Maleeva in the quarter-finals after the Bulgarian beat Chanda Rubin of the United States 6-3 1-6 6-4 in a second round match.

Spanish world number nine Arantxa Sanchez Vicario won a tough first round match, drawing on her vast experience to edge out Slovakia's Karina Habsudova 7-5 6-4.

Sanchez Vicario will now play qualifier Janet Lee of Taipei, who was leading Dutchwoman Brenda Schultz-McCarthy 6-0 1-1 when her opponent retired with a shoulder injury.

On the men's side, defending champion Tim Henman kept his hopes alive of retaining his title as he came back from 3-1 down in the final set to beat experienced Australian Mark Woodforde 6-2, 3-6, 6-4 in their first round match.

The Henman (23), who needs to retain the title to stop him falling further in the rankings after he slipped to 19th on Monday, appeared in trouble in the third set when Woodforde broke to lead 31 in the decisive set under testing, windy conditions.

However, sixth-seeded Henman fought back superbly to dominate the left-handed Woodforde, much to the disappointment of a large home crowd. Henman won the first set at a canter and broke serve to lead 3-2 in the second set.

Woodforde's fighting qualities came to the fore as he won six straight games to win the second set and take an early break in the third. All his good work then came undone, with Henman once again finding his range.

Henman will play Spain's Albert Portas in today's second round.

Australia's newest tennis star, Lleyton Hewitt, pulled off a stunning victory over world number four Jonas Bjorkmann, just days after his historic first ATP tournament win.

Hewitt (16), was the lowestranked and second-youngest player to win an ATP final when he snared the Australian men's Hardcourt title on Sunday.

On the way to that win, in only his second ATP event, he defeated his boyhood hero Andre Agassi.