Andy Murray joins Novak Djokovic among U.S. Open winners

NEW YORK, Sept. 4 (UPI) -- Third-seeded Andy Murray continued his quest to defend his U.S. Open championship Tuesday by knocking off Denis Istomin in four tough sets.

Murray, who already won a Grand Slam title at Wimbledon this year, improved to 17-1 in Grand Slam matches this season by battling to a 6-7 (5-7), 6-1, 6-4, 6-4 win over Uzbekistan's Istomin.

Murray survived despite having his serve broken twice and moved on to the men's quarterfinals.

World No. 1 Novak Djokovic earned a berth in the quarters earlier in the day with an efficient 6-3, 6-0, 6-0 thrashing of unseeded Marcel Granollers. The Serbian star won 25 consecutive points off his serve in the match, which lasted 1 hour, 19 minutes.

Ninth-seeded Stanislas Wawrinka of Switzerland advanced with a 3-6, 6-1, 7-6 (8-6), 6-2 win over fifth-seeded Czech Tomas Berdych. Berdych committed 45 unforced errors.

Mikhail Youzhny of Russia, the 21st seed, came from behind to outlast unseeded former world No. 1 Aussie Lleyton Hewitt 6-3, 3-6, 6-7 (3-7), 6-4, 7-5 in a war of attrition that lasted just under 4 hours. Youzhny broke Hewitt's serve 10 times, while the veteran Hewitt settled for eight breaks in a tough defeat.