Ahead 5-3 in the opening tiebreaker, Murray dropped four points in a row. Serving for the second set at 5-1, Murray hit a volley-lob that Istomin tracked down and, back to the net, sent back with a between-the-legs shot. What should have been an easy tap-in winner became a flubbed volley, and Murray put his hand to his face. But he wound up taking that game, and was on his way to improving to 30-2 over his last five Grand Slam tournaments.
Next for Murray is a quarterfinal against No. 9 Stanislas Wawrinka. A day after his Swiss Davis Cup teammate and friend, Roger Federer, lost, Wawrinka reached the round of eight at the U.S. Open for the second time by beating No. 5 Tomas Berdych, the 2010 Wimbledon runner-up, 3-6, 6-1, 7-6 (6), 6-2 in Louis Armstrong Stadium at night.
In the afternoon, top-seeded Novak Djokovic won 45 of 53 service points while eliminating 43rd-ranked Marcel Granollers 6-3, 6-0, 6-0 in a grand total of only 79 minutes.
Djokovic, who won the 2011 U.S. Open and lost in last year's final to Murray, reached his 18th consecutive major quarterfinal.
Now comes a match against 21st-seeded Mikhail Youzhny, who was two points from defeat but came back to edge two-time major champion Lleyton Hewitt 6-3, 3-6, 6-7 (3), 6-4, 7-5.
All eight men's quarterfinalists are from Europe: Serbia's Djokovic, Britain's Murray, Switzerland's Wawrinka, Russia's Youzhny on one half of the draw, and Spain's Rafael Nadal, David Ferrer and Tommy Robredo, along with France's Richard Gasquet on the other half.
Earlier Tuesday, the fifth-seeded Li needed nearly 2½ hours to get past 24th-seeded Ekaterina Makarova of Russia 6-4, 6-7 (5), 6-2.
On three occasions, Li went up a break in the second set, nearing eventual victory each time, but could not finish things. She then was up 3-0 in the tiebreaker before faltering. Still, she recovered well and closed the match by taking the last four games, then joked that she would grab a bag of chips and enjoy watching Williams against Suarez Navarro.
Regardless of who she'd face next, Li made the point that what would be important is to focus on herself.
"I mean, if you only think about what (your) opponent (is) doing, of course you already lose the match before you come to the court," Li said. "For tennis you have to figure out what you have to do on the court, what you should do."
Li has only won one of nine career matches against Williams heading into Friday.
"Tough, tough opponent," Li said. "But is (a) good challenge to play against her."
The quarterfinals on the other half of the draw are Wednesday: second-seeded Victoria Azarenka of Belarus against 48th-ranked Daniela Hantuchova of Slovakia, and 10th-seeded Roberta Vinci against 83rd-ranked Flavia Pennetta in a matchup between two Italians in their 30s.
While the other women in that section all advanced Monday, Azarenka's fourth-round match against 2008 French Open champion Ana Ivanovic was postponed until Tuesday at 11 a.m. because of rain. Azarenka got off to a slow start, but eventually got going and beat the 13th-seeded Ivanovic 4-6, 6-3, 6-4 in a topsy-turvy match with a combined 16 service breaks and 17 double-faults.
"I think," two-time Australian Open champion Azarenka said, "we're just great returners."
Lamented Ivanovic: "I felt like I could break her, but it was very frustrating that I was losing my serve constantly."
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