Murray neutralizes Isner attack, reaches Cincy quarters
Andy Murray of Scotland hits a return in the match against John Isner on Day 6 at the Western & Southern Open on August 14, 2014 at the Linder Family Tennis Center in Cincinnati, Ohio - by Andy Lyons
Two-time champion Murray saved a pair of match points in the 12th game of the third set as Isner hit the net with errors.
The American was then caught by a passing shot before Murray levelled at 6-6 with an ace.
The Scot then bore down in the decisive tiebreaker, ending the match with his 14th ace.
Murray, in search of his first title since Wimbledon in 2013, has struggled to regain top form since his return from back surgery in January.
The 27-year-old eighth seed advanced with 27 winners and 21 unforced errors, converting only one of his five break chances.
He next faces either second-seeded Roger Federer or Gael Monfils as he tries to add a third Cincinnati title to those he won in 2008 and 2011.
The combined ATP Masters and WTA hardcourt event is the last major tuneup for the US Open, the final Grand Slam of the year that starts at Flushing Meadows on August 25.
Isner, runner-up to Rafael Nadal in Cincinnati last year, was denied another shot at the title, with 21 aces not enough to see him through.
Australian Open champion Stan Wawrinka, the third seed, rallied to beat Croatian Marin Cilic 3-6, 6-0, 6-1 while Italian 15th seed Fabio Fognini recovered to eliminate Lu Yen-Hsun of Taiwan 3-6, 6-3, 6-3.
Frenchman Julien Benneteau advanced into the last eight with a 7-5, 6-1 win over Poland's Jerzy Janowicz.
In women's play, defending champion Serena Williams continued her march through the draw, beating Italian 13th seed Flavia Pennetta 6-2, 6-2.
Also through was world number two and second seed Simona Halep, a 6-4, 7-5 winner against Czech Lucie Safarova.
"Today was much better than my first match here," said Halep, winner in Bucharest last month and runner-up to Maria Sharapova at Roland Garros. "I was happy to win in two sets, since she was up 5-4.
Ukrainian teenager Elina Svitolina, who took out double Wimbledon winner Petra Kvitova in the second round, claimed her second seeded victim with a defeat of Spain's number 15 Carla Suarez Navarro 4-6, 6-4, 6-4.