Vienna: Top-seeded Andy Murray lost a set for the first time in 11 matches as he beat Martin Klizan of Slovakia 6-3, 6-7 (5), 6-0 in the opening round of the Erste Bank Open on Wednesday.
The second-ranked Scot, who won the event two years ago, came off back-to-back tournaments wins without dropping a set in Beijing and Shanghai.
The 29-year-old Wimbledon and Olympic champion, who is closing in on Novak Djokovic's top ranking, took his season record to 66 wins and just nine losses.
Murray's best chance of taking the world number one spot will be to capture the title in Vienna, where he was champion in 2012, as well as next week's Paris Masters.
In the French capital, Djokovic would have to reach the final to stay in pole position.
Australian Open and French Open champion Djokovic has spent a total of 222 weeks at number one in three spells -- including his current run of 121 consecutive weeks.
"I think I could have done better. For sure it was a tough match because Martin goes for his shots a lot," Murray said. "It's not that easy to get into you rhythm. He moves very well, too."
Murray saved three break points in the opening game but his serve came not under threat for the rest of the first set.
He broke Klizan early in the second but dropped serve in an error-strewn game at 4-2. Murray was broken again when serving for the match at 6-5 and lost the tiebreaker from 5-4 up.
"I didn't serve that well the first two sets and that got me in trouble at the end of the second set," the Scot said. "But I managed to serve better and raise my game in the third set."
Murray conceded just two points on own serve in the final set. He will next play Gilles Simon of France.
Also, Karen Khachanov of Russia joined Ivo Karlovic of Croatia and John Isner of the United States in the quarter-finals.
Khachanov saved a set point and won the last five games to defeat Nikoloz Basilashvili of Georgia 6-2, 7-5 to set up a quarterfinal against Karlovic.
The eighth-seeded Croat came back from 5-1 down in the final set to beat Damir Dzumhur of Bosnia-Hercegovina 7-6 (6), 4-6, 7-6 (5).
Isner beat former champion Feliciano Lopez 7-6 (1), 3-6, 7-6 (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 quarterfinal of the season.
In first-round play, Philipp Kohlschreiber of Germany rallied to beat Stephane Robert of France 4-6, 6-3, 7-6 (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.
Updated Date: Oct 27, 2016 11:22 AM