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.
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.