US Open champion Marin Cilic suffered a shock defeat at the Shanghai Masters on Monday, as the Croatian desperately chases points to make the end of season ATP World Tour Finals.
The ninth seed in Shanghai lost to compatriot Ivo Karlovic 7-5, 2-6, 7-6 (7/2) at a pivotal moment in his season.
Cilic is sixth in the race to London and is bidding to qualify for the season finale involving the top eight men's players for the first time.
But Karlovic, who is ranked 31st by the ATP, served impeccably, delivering 20 aces to Cilic's nine in a game which lasted two hours and 16 minutes.
"It's a gamble with him," said Cilic.
"He goes all in or nothing. It's always difficult. He was serving really well towards the end of the match. I couldn't get into his serve. It can happen with Ivo. It's always tricky.
"The game, I'm feeling it's on the spot. Though I lost today, it's unfortunate, bad luck, but I'm going with positive energy into the next weeks to finish the season well."
Cilic lost his quarter-final in the China Open last week to another player bidding for a place in the end of season finale, Britain's Andy Murray.
Elsewhere in Shanghai, American John Isner fired 22 aces in his 7-6(7/5), 1-6, 6-3 victory over Pablo Andujar of Spain to make the second round.
The 13th seed in Shanghai lost to world number six Czech Tomas Berdych in the quarter-finals in Beijing last week, but was content with his performance in the Chinese capital and in Shanghai.
"Beijing was great last week," said Isner.
"I had a pretty good tournament. Got my tail kicked in the quarters. But I'm playing pretty well. I'm still alive here.
"I'm glad I'm not on a flight home tomorrow (Tuesday). It's all good. Playing better away from the States is something I'm always trying to do and I feel like it's something I've done so far."
Meanwhile, Richard Gasquet of France defeated compatriot Jeremy Chardy 7-6(7), 6-2 to set up a second-round clash with Berdych, the eventual runner-up at the China Open.
South African Kevin Anderson, the 16th seed in Shanghai, overcame British number two James Ward 3-6, 6-3, 6-2.
The $6,521,695 Shanghai Masters, an ATP World Tour Masters 1000 event, concludes on Sunday.