Andy Murray has survived a first round scare at the ATP tournament in Vienna in his bid to chase down Novak Djokovic in the world rankings.
The in-form Murray had won 24 consecutive sets before Slovakia's Martin Klizan took a tie-break off him in Vienna.
But the Wimbledon and Olympic champion cruised through the decider to win 6-3 6-7 (5-7) 6-0 in two hours and 22 minutes and set up a last-16 date with Frenchman Gilles Simon.
Murray admitted he had his sights on taking over top spot in the world rankings for the first time after back-to-back wins at the China Open and the Shanghai Masters.
But Murray - champion in Vienna two years ago - was troubled by an opponent he was meeting for the first time and who had missed a chunk of the season with a foot injury.
The world No.35 had three break points in the opening game, but Murray denied them all and went on to take the first set in 42 minutes.
Murray looked on course for victory when serving at 4-3 and then 6-5 in the second set.
But Klizan's ability to produce forehand winners saw Murray lose only his sixth tie-break out of 25 this year.
There had been some rare unforced errors in Murray's game, but they were eliminated in a one-sided third set which was somewhat suitably settled in the end by a Klizan double fault.
Earlier, Isner beat former champion Feliciano Lopez 7-6 (7-1) 3-6 7-6 (7-2).
The 27th-ranked American failed to break the Spaniard's serve and missed a match point at 5-4 and again at 6-5 in the final set. Isner dominated the tiebreaker and closed out the win on his 31st ace.
"The match was so close. My serve helped me a lot," said Isner, who will play Murray or Simon in his fifth quarter-final of the season.
In first-round play, Philipp Kohlschreiber of Germany rallied to beat Stephane Robert of France 4-6 6-3 7-6 (7-3), and Albert Ramos-Vinolas of Spain advanced with a 6-2 6-2 win over Italy's Fabio Fognini, who had not lost to Ramos-Vinolas in eight previous matches.
