Roberto Bautista Agut suffered a shock first-round exit against Chung Hyeon of South Korea at the Shanghai Rolex Masters.
The Spanish world number 13 and tournament's ninth seed, who was beaten in last year's final by Andy Murray, lost 6-4 6-3 in straight sets against the 21-year-old from South Korea, who broke Bautista Agut in his final two service games to secure victory.
World number 38 Benoit Paire is also out after the Frenchman lost 6-4 6-4 to 19-year-old American Frances Tiafo and Greek qualifier Stefanos Tsitsipas beat Russian Karen Khachanov 7-5 6-3.
Juan Martin Del Potro, though, avoided an upset when the 16th seed from Argentina came through in three sets, 6-2 3-6 6-4, against Nikoloz Basilashvili of Georgia.
British number two Kyle Edmund defeated Jiri Vesely 6-3 6-2 to set up a second-round match against fourth seed Marin Cilic.
Spain's Feliciano Lopez, the world number 34, beat big-serving Croatian Ivo Karlovic 7-6 (7/1) 7-6 (8/6) while compatriot Albert Ramos-Vinolas came from behind to see off Portuegse Joao Sousa 3-6 6-3 7-5.
Richard Gasquet will play Chung after the Frenchman recovered from losing the opening set tiebreak to beat Dutchman Robin Haase 6-7 (11/13) 7-5 6-2.
Monday's late matches saw Andrey Rublev of Russia come through in three sets 6-1 4-6 6-1 against Bosnian Damir Dzumhur, while Chinese player Di Wu knocked out Frenchman Jeremy Chardy, beating the world number 83 in straight sets, 7-6 (7/2) 6-2.