Defending champion Grigor Dimitrov lost 6-4, 7-6 (5) to Gilles Muller of Luxembourg in the second round at Queen's Club on Thursday.
The sixth-seeded Bulgarian won the title last year by defeating Feliciano Lopez, but he has failed to reach a final since. He was bidding to successfully defend a title for the first time in his career.
In a match of few rallies in which Muller had 19 aces and Dimitrov 15, the Bulgarian double-faulted at 5-4 in the first set and then made a forehand error to give Muller the only break of the match and a 1-0 lead.
Dimitrov saved a break point at 2-1 in the second with a backhand winner down the line, but Muller was stronger in the tiebreaker.