New York - Didn't take long for Serena Williams to show her fourth-round opponent at the U.S. Open where things were headed.

"The first point of the whole match," 82nd-ranked Andrea Hlavackova explained, "when I served, and she returned, like, a 100 mph forehand return, I was like, "OK, I know who I'm playing. You don't have to prove it to me. I know."'

Monday's match was less than 15 seconds old. It might as well have been over.

Dominant from the moment she ripped that return of an 88 mph second serve, forcing Hlavackova into an out-of-control backhand that sailed well long, to the moment she powered a 116 mph service winner on the last point, Williams extended her 2½-month stretch of excellence with a 6-0, 6-0 victory to get to the quarterfinals at Flushing Meadows.

Those two big zeros pretty much tell the story; it's the fifth time in her career Williams won with what's commonly called a "double bagel." Some other impressive numbers: Williams won 60 of 89 points, built a 31-9 edge in winners and improved to 23-1 since losing in the first round of the French Open. That run includes singles and doubles titles at both Wimbledon and the London Olympics.

"My coach warned me to not go on the court and play for a score," Hlavackova said, by which she meant just trying to keep it as close as possible. "I was in the match. I was trying to figure out how to win. And when it was, like, 6-love, 4-love, 30-love, I was thinking, "Well, I'm not playing for a score, but one game wouldn't hurt."'

Oh, well.

Next for the fourth-seeded Williams, who won the U.S. Open in 1999, 2002 and 2008, is a match against former No. 1 Ana Ivanovic, who reached a Grand Slam quarterfinal for the first time since winning the 2008 French Open by defeating 55th-ranked Tsvetana Pironkova of Bulgaria 6-0, 6-4.

Williams is 3-0 against Ivanovic, including a straight-set victory in the fourth round at Flushing Meadows last year.

In the semifinals, the Williams-Ivanovic winner will meet either No. 10 Sara Errani or No. 20 Roberta Vinci, doubles partners who both eliminated higher-seeded women Monday and now face the uncomfortable prospect of trying to beat a best buddy.

"Our friendship won't change, no matter what, no matter who wins," said Vinci, noting that she expects they'll have dinner together, as usual, Monday and today. "It definitely won't be an easy match from a mental perspective. We know each other well. We practice together often. We play doubles together. We know everything about each other."

Errani and Vinci teamed up to win the French Open doubles championship in June, and now one of them is going to be the first woman from Italy to play in the U.S. Open semifinals since the professional era began in 1968.

Errani, the runner-up to Maria Sharapova at the French Open, got past No. 6 Angelique Kerber of Germany 7-6 (5), 6-3.

Reaching her first singles quarterfinal in 32 career Grand Slam tournaments, Vinci stunned No. 2 Agnieszka Radwanska 6-1, 6-4.

"I really had the worst day," said Radwanska, who could have moved up to No. 1 in the WTA rankings by reaching next weekend's final. "She really mixes it up - a lot of slice, then suddenly hitting very well from the forehand side, then kick serve, drop shots, volleys as well, coming to the net. It's really tough because she really had an answer for everything."

Radwanska could sympathize with Hlavackova's plight, having lost to Williams in the Wimbledon final.

"When she's on fire, you can't do anything about it. It doesn't matter what you try to do, it's going to be a winner," Radwanska said. "It's just Serena. She's a great champion."

So is No. 1-seeded Roger Federer, of course, owner of a record 17 Grand Slam titles, including five at the U.S. Open. As he attempts to add to those numbers, Federer got some extra rest Monday, because his fourth-round opponent, 23rd-seeded Mardy Fish of the United States, withdrew hours before their scheduled match for precautionary reasons, citing medical advice.

No. 12 Marin Cilic of Croatia put together a 7-5, 6-4, 6-0 victory over 50th-ranked Martin Klizan of Slovakia, the last left-hander and unseeded man remaining, and will take on the winner of Monday night's match between London Olympic gold medalist Andy Murray and No. 15 Milos Raonic of Canada.

The 30-year-old Fish missed about 2½ months this season because of an accelerated heartbeat and had a medical procedure in May.

"We are not 100 percent sure what the issue is and if it is related to his previous issues," Fish's agent, John Tobias, wrote in an email to The Associated Press. "Mardy is fine and will return home to L.A. tomorrow. This was strictly precautionary and I anticipate that Mardy will play in Asia this fall."