Roger Federer's comeback cut short in Stuttgart as 18-time Grand Slam champion loses to veteran Tommy Haas

Federer was making his return after sitting out of the clay-court season but fell to a shock 2-6 7-6(8) 6-4 loss to 39-year-old German veteran Haas

Federer slipped to a shock defeat against the Dutch veteran (Getty)

Roger Federer’s comeback got off to the worst possible start on Wednesday, as the 18-time Grand Slam champion slumped to a 2-6, 7-6(8), 6-4 loss to German veteran Tommy Haas at the Mercedes Cup in Stuttgart.

Federer, 35, made a stunning start to the 2017 season, winning the Australian Open before completing the Sunshine Double by winning titles at Indian Wells and Miami. He then decided to skip the entire clay-court season in order to stay fresh for Wimbledon, which he is attempting to win for an unprecedented eighth time.

He made a fine start to his match against 39-year-old Haas, winning the first set and holding a match point in the second.

But Haas, who is due to retire at the end of the current season, fought back to win in three sets. He will now play against his compatriot Mischa Zverev in the quarter-final.

“I am speechless, of course,” Haas said in front of a home crowd. “Playing Roger is always special, especially on grass. 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.”

Haas is through to the quarter-final of his home tournament (Getty)

Federer grabbed an early break as his attacking topspin backhand, which had served him so well earlier this year, was again on display.

Haas, a former world number two but currently at 302, was broken again as the Swiss served out the set on his first opportunity after 22 minutes and looked to be cruising to a comfortable victory.

But Federer lost his focus, allowing Haas to find a way back into what until that point had been a one-sided encounter.

The German was left "speechless" by his victory (Getty)

The world number five saved four set points and wasted a match point as the German forced a third set, where he broke his opponent early on.

He held on to his advantage and sealed a memorable win with his second match point, improving his record against Federer to 4-13.

Federer has now lost twice this year (Getty)

“This is my last phase, so the emotions are different from what they would have been a few years ago,” the World No 302 added. “This is sport. Unfortunately, one person has to lose and lately a lot of the time it has been me.

“I was happy to be out there in front of a German crowd playing against a friend, an idol, a legend, the greatest ever. It’s fantastic.”

The defeat was Federer’s first since his three-set loss to Russian Evgeny Donskoy at the Dubai Duty Free Tennis Championships, in February. Federer has lost just two matches all year.

Additional reporting by Reuters.