Serena Williams is still waiting for her first true test of this US Open.

The defending champion's fourth-round match was competitive for all of four games yesterday, before Williams took 10 in a row to beat No. 22-seeded Daniela Hantuchova of Slovakia 6-2, 6-0.

Williams has reached the quarterfinals at 11 of the last 12 major tournaments and won the title at three of the last four.

With the score two-all against Hantuchova, Williams hit three aces to hold for a 3-2 lead - and she wouldn't lose a game the rest of the way.

She has won all eight sets she's played this year at Flushing Meadows.

"I tried to relax," Williams said after improving to 8-1 against Hantuchova over their careers.

"Sometimes I'm such a perfectionist I place too much stress on myself. I was like, 'Serena, relax!"'

She finished with eight aces and a 27-9 advantage in winners in yesterday's first match in Arthur Ashe Stadium.

Highest-ranked woman

Her older sister Venus, the 2000-01 US Open champion, was scheduled to face 2005 champion Kim Clijsters in a fourth-round match later yesterday.

No. 1 Dinara Safina was upset in the third round Saturday night, meaning No. 2 Williams is the highest-ranked woman left in the field.

Safina will remain atop the rankings, though, even if Williams were to win a second consecutive US Open title and fourth overall.

Safina was disappointed by her loss - and angry about her court assignment. Bumped out of Ashe for her late-night match on Saturday because of time concerns, Safina was beaten by 72nd-ranked Petra Kvitova of the Czech Republic 6-4, 2-6, 7-6 (5).

Ahead 6-5 in the third set, Safina had three match points but lost them all.

She blamed herself for that. But she also complained about the court change - both because she felt disrespected by the move and because she thought organisers could have handled the situation better.

"From my side, I can say, I'm No. 1 player in the world, why did they move me?" Safina said.

Her match was shifted from Arthur Ashe Stadium to the smaller Louis Armstrong Stadium because of a late-running afternoon session.

Night session

The original schedule called for Safina-Kvitova to open the night session in Ashe, followed by a men's match between Tommy Robredo and James Blake. But because things were running late, organisers opted to play those matches simultaneously.

"They just told us, 'We're switching you to Armstrong'. And basically that's it," Safina said. "And I think it's very unfair."

Kvitova didn't seem to mind. Asked after her victory about the court change, she said: "Yeah, doesn't matter for me. I'm not (a) star, so ..."

Her win over Safina ended at 12:50 a.m., about 10 minutes before Robredo wrapped up his three-set victory over Blake next door.