NEW YORK - Serena Williams fought off the wind, along with brief flurries of effectiveness from her opponent, to advance to the quarterfinals of the U.S. Open on Monday with a 6-3, 6-4 victory over Ana Ivanovic.

Williams closed out the match with four straight serves that Ivanovic couldn't get back - clocked at between 99 and 111 mph in a blustery Arthur Ashe Stadium that had both players fighting with their tosses and topspin all day.

"I didn't even go for winners at any point," said Williams, who hit only 16. "I just tried to get it over because it was so windy. It was definitely tough."

Seeded only 28th after missing big chunks of the last two years with injuries to her foot, Williams nonetheless improved to 16-0 this year on hardcourt. She has yet to drop a set at the U.S. Open and now finds herself in her first Grand Slam quarterfinal since last year at Wimbledon, when she won her 13th major title.

With each win at Flushing Meadows, she makes a stronger case that the "28" before her name at this tournament is only a number. When healthy, she might be the best in the world.

"I don't know if I'm the best or not," she said. "I believe I am and I think a lot of other girls, women in the locker room, believe they are, too, as they should. I don't think anyone should go out and say that they're not."

In men's play, the highest-ranked American, eighth-seeded Mardy Fish, lost to 11th-seeded Jo-Wilfried Tsonga 6-4, 6-7 (5), 3-6, 6-4, 6-2.

Fish was trying to make it past the fourth round at Flushing Meadows for the second time. But his play dipped in the fourth set, and he was treated by a trainer for a right leg problem before the fifth.

"Old age, I guess," said the 29-year-old American.

The top seed on the men's side, Novak Djokovic, opened his fourth-round match with a thrilling 16-14 first-set tiebreaker win over No. 22 Alexandr Dolgopolov. Things got easier from there in a 7-6 (14), 6-4, 6-2 victory.

"This is one of the longest tiebreaks I ever played," Djokovic said. "It was certainly exciting to be part of it. But, you know, I knew that I needed to win that set."

Djokovic saved four set points and finally closed out the set on his sixth chance. When Dolgopolov missed a forehand to give Djokovic the set, Djokovic pumped his fist and put his hands to his ears to take in the applause from the fans at Louis Armstrong Stadium, which sees fewer and fewer headliners as the tournament progresses and the big matches move into Ashe.

In the women's match, 16th-seeded Ivanovic, the 2008 French Open champion and a one-time world No. 1, came in on a rebound after a slide down the rankings into the 60s. At times against Williams, flashes of the old Ivanovic showed, especially when she drew back to 3-3 in the first set after dropping the first three games in eight minutes.

Taking the ball early, unafraid to step inside the baseline to return Williams' second serves, Ivanovic was the aggressor during that portion and in parts of the second set when she tried, unsuccessfully, to make up the break she lost in the first game.

But she couldn't overcome eight double faults, including three while serving at 3-4 in the first set, and didn't have an answer for Williams who was less aggressive (16 winners to 20 for Ivanovic), but more consistent (14 unforced errors to 29) and also had more bite on her serve (nine aces).

"My serve broke down a little more than hers," Ivanovic said. "I still created lots of opportunities and I felt I was stepping up a lot and I just felt that was the biggest difference today."

Next up for Williams is No. 17 Anastasia Pavlyuchenkova, who made it to her second Grand Slam quarterfinal with a 5-7, 6-3, 6-4 win over former French Open champion Francesca Schiavone.

The three-set match included 16 service breaks over 31 games and, appropriately enough, ended when Pavlyuchenkova hit a forehand winner on match point to break Schiavone's serve for the ninth time. The players combined for 21 double-faults.