Federer’s self-imposed hiatus saw him sit out the entire clay court season with an eye on challenging for the Wimbledon and US Open titles. But the 35-year-old showed noticeable signs of rust in his first game back, struggling for consistency with his ground strokes against an inspired Haas.
"I thought it was a typical grasscourt match today. Rallies weren't very long and it was decided on a serve there or a return here," Federer told reporters.

Highlights: Roger Federer stunned by Tommy Haas in Mercedes Cup second round

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"If you don't take your chances like I didn't, leading a set and a break, you really only have yourself to blame at the end. You have to acknowledge the fact that he was a bit better. It's quite frustrating, but that’s the way it goes sometimes.
"I definitely was not as sharp as I was hoping to be in maybe the big moments, or the moment when I had the lead and where I feel I should have been cruising from that moment on."

Haas: 'I kind of, sort of, shocked myself!'

Federer raced to a one-set lead in just 23 minutes in front of a packed crowd. But the watching fans were treated to a more competitive second set in the baking afternoon sun as the two elder statesmen shared a break apiece on the way to a tiebreak.
Haas eventually took the tiebreak, after having a string of set points saved, and carried that form into the deciding set, holding his nerve to serve out for a 6-4 win to seal a deserved win in front of his home crowd.
The 39-year-old Haas, ranked 302nd and in his farewell season, proved more consistent, eventually setting up a quarter-final clash against countryman Mischa Zverev. With another victory Haas, the lowest-ranked player to beat Federer since world number 407 Bjoern Phau in 1999, will become the oldest player to reach a tour semi-final since 40-year-old Jimmy Connors in 1993 in San Francisco.
"I am speechless, of course," Haas said. "There are a lot of emotions when you are playing a close friend. "When you come into the second set, save a match point and then come back, that always makes you very happy about a win. I am shocked myself."

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Mercedes Cup Second Round Results

Tommy Haas 2-6 7-6 6-4 Roger Federer
Mischa Zverev 7-6 6-2 Yannick Hanfmann
Philipp Kohlschreiber 7-6 5-7 7-6 Steve Johnson
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