Roger Federer said he was not surprised to lose to Tommy Haas at the Mercedes Cup in Stuttgart on Wednesday in his first match back after almost three months out of action.
Haas, 39, now ranked at 302 in the world, beat the Swiss maestro 2-6, 7-6 (10/8), 6-4 for just his fourth victory in 17 meetings with his close friend.
Federer, who had been resting since March, skipped the entire clay court season in order to prepare for a crack at his eighth Wimbledon title.
It marked just his second defeat of the year after he won the Australian Open in January – his 18th major title.
"I'm not so shocked to have lost. I knew I could lose against Tommy," said Federer.
"He was so good at the end and if I am happy for anybody, then it's for Tommy."
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Haas, a former world number two, saved a match point and seven break points in the deciding set before going on to win in a battle of the veterans.
The soon-to-retire Haas made his first quarter-final since Rome in 2014. With the two players' combined age adding up to 74, it was the oldest match on the tour since 1982.
Speaking after his surprise victory, Haas said: "It's always special to play a close friend like Roger on grass, always lots of emotion."
"I saved a match point in the second set so was able to play more freely after that. I even shocked myself," he added.