STUTTGART, Germany (AFP) - Roger Federer uncharacteristically failed to nail two match points as Dominic Thiem stunned the top seed on the grass in the rain-disrupted Stuttgart semi-finals on Saturday.

The Swiss legend, who is returning from a back injury that ruled him out of the French Open, saw his preparations for Wimbledon unravel as Thiem battled through 3-6, 7-6 (9/7), 6-4.

The third seed Thiem, a Roland Garros semi-finalist who moved into the ATP top 10 only this week, will play for his fourth title of the season on Sunday against Juan Martin del Potro or Philipp Kohlschreiber.

"You're always a little disappointed when you lose with match points," Federer said after surrendering the match points that would have given him a straight-sets victory. "But losing any match is not that great."

The contest was halted twice for a total of an hour by rain showers, but Federer - who turns 35 in August - did not attempt to make any excuses.

"It was a good match from both sides in tough conditions," he said.

The 17-time Grand Slam champion, who eased through the first set, put on a charge after falling behind 5-1 in the second set, denying the Austrian as he served to win the set.

Federer forced the set into a tiebreaker, where he had his match-point chances, but Thiem held on and then made it a set apiece.

The third set was interrupted after the fifth game with Thiem up a break after Federer missed a volley at the net.

"We both had our chances throughout," Federer said. "Coming back in the second set was good but it was tough to lose in the breaker.

"I can play better, but he came up with some good shots when he needed them." Thiem called the victory one of his best, saying beating an injured Federer on the clay in Rome last month did not compare.

"The way I played and felt on court was one of the best feelings I've had in tennis. For sure this was one of my best matches - I didn't know I could play so well on grass," said the winner.

"In Rome he was not fit and just testing to see if he could play Paris. The win was nice but it didn't mean so much like now. This is probably his best surface."

Federer, who set a record in the quarter-finals as he notched his 1,072nd career match win to stand second behind Jimmy Connors in the all-time table, last played a final in January in Brisbane, losing to Milos Raonic.

His season has been interrupted by back pain which forced him to miss much of the clay season including the French Open.

His last title came in the autumn at home in Basel.