Dominic Thiem's losing run was extended to three matches as Steve Johnson pulled off an upset victory against the Austrian in the first round of the Japan Open.
Thiem was seeded second in Tokyo, but the world number seven suffered a 4-6 7-6 (7-5) 6-4 loss on Tuesday.
After failing to open up any match-point opportunities in the second-set shootout, Thiem had to fend off three break points in the opening game of the decider, but Johnson moved ahead in game five.
The American was made to fight hard in game eight by Thiem, but once the world number 46 held firm, it sapped the fight from his opponent.
Third seed Milos Raonic had no such troubles as he returned to action with a straight-sets win over Viktor Troicki.
Raonic had missed the previous seven weeks with a wrist injury, but looked good in his attempts to match his three runs to the final in Tokyo, beating Serbian Troicki 6-3 6-4 in 72 minutes.
"The injury developed over the past 10 years, the pain had been manageable, but I hit a breaking point and the pain was not tolerable," said Raonic.
"The surgery was not structural, more taking away swelling. It's been frustrating, having more than a dozen injuries that have kept me away from tennis."
Yuichi Sugita is next up for Raonic after Benoit Paire retired when a set down to the Japanese.
Sixth seed Sam Querrey could not match his compatriot Johnson's exploits, going down 6-4 7-6 (7-2) to Richard Gasquet. Lu Yen-hsun is the Frenchman's next opponent after beating Taro Daniel.
Matthew Ebden scored a 6-4 6-3 win over veteran Ivo Karlovic in a match that started on Monday before being halted by rain, while eighth seed Diego Schwartzman will meet Bernard Tomic in round two after beating Donald Young 6-2 7-5.