Tim Henman's comeback from a knee injury ended with defeat by Spain's Feliciano Lopez at the round-robin Las Vegas Open on Thursday.
Henman slumped after a strong showing in the first set
Henman lost 6-7 (1-7) 6-3 6-3 to the world number 93, who won both his Group Seven matches to reach the last eight.
After storming through the tie-break the Briton never threatened again, and he gave up a break in each of the next two sets to hand Lopez the match.
The Las Vegas event was Henman's first outing since last October.
Henman had beaten Nicolas Mahut in his first match in Nevada but had needed to defeat Lopez to advance.
Third seed Marat Safin won his group with a hard-fought 4-6 7-6 (7-4) 7-5 win over compatriot Igor Kunitsyn
And second seed Lleyton Hewitt continued his return from injury by defeating Thomas Johansson 4-6 7-5 6-2 to secure a quarter-finals place.
"I don't think my movement is probably as sharp as I'd like it to be," said Hewitt.
"But that's just from not playing matches, and being a bit careful on the hamstring."
Former French Open champion Gustavo Kuerten's tournament ended with a 6-4 6-4 loss to Jan Hernych.
Top seed and defending champion James Blake went through after winning his second match against Juan Martin del Potro, who retired at 6-1 3-1 down.
Blake, Del Potro and Russia's Evgeny Korolev had each won a match in Group One but the ATP decided the American should go through.
Britain's James Auckland and Ashley Fisher of Australia reached the semi-finals of the doubles with a 6-4 1-6 10-8 victory over Americans Paul Goldstein and Jim Thomas.