Serena Williams didn't drop a game in advancing to the quarterfinals of the U.S. Open.

Roger Federer moved on with even less work when Mardy Fish pulled out of their fourth-round match Monday for precautionary reasons.

Fish missed about two months this season because of an accelerated heartbeat and had a medical procedure in May. His agent, John Tobias, said "we are not 100 percent sure what the issue is and if it is related to his previous issues."

"I was reluctant to do so, but am following medical advisement," Fish said about his withdrawal in a statement. "I had a good summer and look forward to resuming my tournament schedule in the fall."

The 30-year-old American's third-round match against Gilles Simon went five sets, lasting more than 3 hours and ending after 1 a.m. Sunday. Afterward, the 23rd-seeded Fish did not appear at a news conference. Tournament officials said he was getting treatment, but didn't give specifics.

Williams beat 82nd-ranked Andrea Hlavackova 6-0, 6-0 in 57 minutes. It was the first time in 62 career victories at Flushing Meadows that the three-time U.S. Open champion enjoyed a "double bagel."

Williams had 31 winners and just seven unforced errors.

Hlavackova, a 2011 French Open doubles champion, had never been past the second round in singles at a major tournament before this year.

Williams will next face Ana Ivanovic, who is finally back in a Grand Slam quarterfinal more than four years after her French Open title.

The 12th-seeded Serb beat Tsvetana Pironkova 6-0, 6-4. Ranked No. 1 in the world in 2008, Ivanovic has struggled with injuries and her confidence since.

"There were some good moments and some tough ones, but it's a process. I understand it better now," she said. "There are times when you realize that it's exactly what it is: It's a process. You can't have everything at the same time. You have to be consistent on practice courts and then in the matches."

After failing to serve out the match at 5-3 in the second set, Ivanovic broke the 55th-ranked Bulgarian at love in the next game to clinch the victory.

Pironkova, a 2010 Wimbledon semifinalist, went up a break twice in the second set only for Ivanovic to immediately break back. There were seven service breaks in 10 games in the set.

Pironkova held serve just once in the match. Ivanovic had more success on Pironkova's first serve than her own. The Serb won 65 percent of the points when her opponent got in her first serve, compared with 61 percent when her own first serve landed in.

Federer, in his 34th consecutive quarterfinal at a major tournament, will meet sixth-seeded Tomas Berdych, who beat 11th-seeded Nicolas Almagro 7-6 (4), 6-4, 6-1 in 2 hours.

"At least good that I didn't stay that long on court, that I was able to save some energy, as well," Berdych said.