SHANGHAI (AP) -- U.S. Open champion Marin Cilic was eliminated in the first round of the Shanghai Masters, losing to Ivo Karlovic 7-5, 2-6, 7-6 (2) Monday.
Karlovic had 20 aces and won 85 percent of points on his first serve to prevent Cilic from securing his spot at the ATP finals in London.
Cilic is in the sixth spot for the eight-man field, with several players not far behind as the season winds down.
The ninth-seeded Croat said he was feeling drained last week when he played his first tournament following his win in New York at the China Open in Beijing, where he lost in the quarterfinals to Andy Murray.
''This week I'm feeling better. The game, I'm feeling it's on the spot,'' Cilic said. ''(At the) beginning of the tournaments I'm going to have to be more focused to go through first few matches. Happened the same in New York. I played a bit more rusty in the first few matches, then afterwards just everything was flowing.''
Also, 10th-seeded Grigor Dimitrov of Bulgaria was 4 for 4 on break chances in beating Denis Istomin of Uzbekistan 6-3, 6-3.
Dimitrov, a Wimbledon semifinalist this year, is one of the players trying to make up points to qualify for London. He is currently in 11th place, 335 points behind Milos Raonic in eighth.
John Isner of the United States hit 22 aces and saved six of eight break points to beat Pablo Andujar of Spain 7-6 (5), 1-6, 6-3.
Isner, seeded 13th, is coming off a quarterfinal appearance at the China Open, where he lost to finalist Tomas Berdych.
''I'm still alive here. I'm glad I'm not on a flight home tomorrow,'' Isner said. ''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.''
Jack Sock came back from 0-3 down in the final set to defeat Bernard Tomic of Australia 7-6 (4), 6-7 (4), 6-4 in a night match. Tomic had 15 aces in the losing effort.
Kevin Anderson of South Africa, seeded 16th, rallied to beat British qualifier James Ward 3-6, 6-3, 6-2; Richard Gasquet beat Jeremy Chardy 7-6 (7), 6-2; Yen-Hsun Lu of Taiwan defeated Marcel Granollers of Spain 6-3, 6-7 (5), 6-2; and Tunisian qualifier Malek Jaziri beat Donald Young of the United States 7-5, 6-4.
Top-ranked Novak Djokovic, Roger Federer, Rafael Nadal and Murray head the field at the tournament, which features the top 10 players in the rankings. Djokovic, Federer and Nadal received first-round byes; Murray plays Russian qualifier Teymuraz Gabashvili in his opening match on Tuesday.