Andy Murray's 19-match winning streak on grass came to an end with defeat by Radek Stepanek at Queen's Club.
Murray, the defending champion, missed eight set points in the first set before going down 7-6 (12-10) 6-2 to the 35-year-old Czech.
It was the Briton's first defeat on grass since the 2012 Wimbledon final against Roger Federer.
Murray's next competitive appearance will be when he opens play at Wimbledon as the defending champion on 23 June.
The 27-year-old's hopes of gaining extra match time on the grass, and ideally winning a fourth Aegon Championships title, floundered on a sweltering afternoon in London.
He will now hope to put in some valuable work with Amelie Mauresmo, who was named as his new coach on Sunday.
"I thought the first set was a pretty high standard," said Murray. "I thought he served pretty well throughout the whole match.
"I have only got myself to blame that I lost the first set. I don't know how many set points I had, but quite a lot of them were on my serve.
"On this surface especially, you shouldn't really be losing sets like that. For me, that's what's disappointing about the match.
"Then unfortunately I got broken in the first game of the second set. I couldn't quite get it back. The tie-break was tough."
Stepanek always had the credentials to test Murray as a former semi-finalist at Queen's and quarter-finalist at Wimbledon, and a fixture in the world's top 50.
It was against Stepanek at Wimbledon in 2005 that the 18-year-old Scot first made his breakthrough in terms of national attention, when he beat the then 14th seed in the second round.
The Czech had won only one of their five meetings since then but chose a high-profile occasion to take his revenge.
Murray had his chances, breaking serve in the first game only to hand it straight back, and letting a 6-2 lead slip in the tie-break, with a double fault and two unforced errors contributing.
Stepanek saw off four more set points before grabbing his opportunity, thumping away a forehand volley on his second set point.
Murray could make little impact on an increasingly confident opponent, winning just four of 42 points on the Czech's first serve.
With his own second serve vulnerable, he remained under pressure.
Any hope of a fightback from the champion disappeared in game seven as a Stepanek return on break point clipped the net and dribbled over, leaving Murray stranded and facing a double-break.
There was no more luck required as Stepanek saw out the final service game, and Murray had once again come up short when defending the title he won in 2009, 2011 and 2013.
Top seed Stanislas Wawrinka beat Sam Querrey 6-2 6-2 to set up a quarter-final against Marinko Matosevic, who beat fifth seed Jo-Wilfried Tsonga 6-2 6-4.
Second seed Tomas Berdych joined his compatriot Stepanek in the last eight with a 7-6 (7-2) 6-4 victory over Adrian Mannarino.
Berdych, the world number six, next plays Spanish 10th seed Feliciano Lopez, a 7-6 (7-3) 6-7 (4-7) 7-6 (7-3) winner over Kenny De Schepper.
Meanwhile, defending champion Daniela Hantuchova and second seed Sam Stosur both lost in the third round of the Aegon Classic in Birmingham, but top seed Ana Ivanovic went through in straight sets.
Kimiko Date-Krumm, the 43-year-old Japanese, beat Hantuchova 6-4 6-0 and Casey Dellacqua overcame Stosur, her fellow Australian, 2-6 6-2 6-2.