NEW YORK Andy Murray stumbled at the end of the first set, dropping four points in a row. He also briefly faltered at the end of the second set, missing a simple volley after his opponent's between-the-legs shot.
But the defending U.S. Open champion steadied himself and beat 65th-ranked Denis Istomin of Uzbekistan 6-7 (5), 6-1, 6-4, 6-4 on a windy Tuesday night.
"Might not be that easy to see from the side, but in the court, there was a strong breeze," the third-seeded Murray said. "We were both struggling with the timing, but I thought we played some entertaining points. Sometimes when it's very breezy like that, you can get some fun points."
Murray's quarterfinal opponent is ninth-seeded Stanislas Wawrinka, who eliminated No. 5 Tomas Berdych 3-6, 6-1, 7-6 (6), 6-2.
After leading 5-3 in the opening-set tiebreaker, Murray ran into trouble.
First, Istomin hit a forehand winner to close a 27-shot exchange. Then Murray, distracted before his service toss by a shout from the stands, double-faulted. A forehand into the net by Murray gave Istomin a set point. And Murray ended the set by pushing a backhand wide on a 12-stroke point.
Istomin shook his racket and screamed. But most of the second set was no cause for celebration. Istomin got broken right away and trailed 3-0 after 12 minutes.
In the third set, Murray displayed more shakiness, going up a break, only to lose serve to make it 3-all.
But he broke right back for a 4-3 edge and was back on his way to a victory.
Earlier, top-seeded Novak Djokovic won 45 of 53 service points while eliminating 43rd-ranked Marcel Granollers of Spain 6-3, 6-0, 6-0.
Djokovic, who won the 2011 U.S. Open and lost in last year's final to Murray, faces 21st-seeded Mikhail Youzhny in the quarters. Youzhny was two points from defeat but came back to edge Lleyton Hewitt of Australia 6-3, 3-6, 6-7 (3), 6-4, 7-5.
On the women's side, defending champion Serena Williams was close to perfect in a 6-0, 6-0 win over 18th-seeded Carla Suarez Navarro of Spain. It was the first "double bagel" in a quarterfinal at Flushing Meadows since 1989, when Martina Navratilova did it to Manuela Maleeva.
Williams won 53 of 71 points in the 52-minute match.
Asked whether she'd describe her performance as close to flawless, Williams said, "Of course not. I played good, though. I was just more focused than anything. You know, I like to believe there is room for improvement."
That might be bad news for her semifinal opponent, fifth-seeded Li Na, who beat 24th-seeded Ekaterina Makarova 6-4, 6-7 (5), 6-2.
In a fourth-round match postponed a day by rain, second-seeded Victoria Azarenka overcame a slow start to defeat 13th-seeded Ana Ivanovic 4-6, 6-3, 6-4. The topsy-turvy match included a combined 16 service breaks and 17 double faults. Azarenka today meets Daniela Hantuchova of Slovakia.
Altamirano wins in juniors Yuba City's Collin Altamirano, a wild card, beat 12th-seeded Karen Khachanov of Russia 6-4, 6-4 in the second round of boys singles.