Halle Westfalen - Japan's Yuichi Sugita shocked Dominic Thiem to send the Austrian out in the second round of the ATP Halle grass court event on Wednesday, 6-2, 7-5,
Third seed Thiem, French Open runner-up to Rafael Nadal earlier this month, was joined on the sidelines of the pre-Wimbledon tuneup by Japanese seventh seed Kei Nishikori, a 6-2, 6-2 loser to Karen Khachanov.
In 2017, the 36th-ranked Russian also beat Asia's top player in the second round at Halle, when Nishikori was forced to quit with a hip injury.
Thiem switched over to grass this week and came to the court with a first-round win in northern Germany at a venue styled as a smaller replica of the All England club's iconic Centre Court at Wimbledon.
"Sugita played really well, and gave me little chance to attack," Thiem said. "I was missing a few percentage points on my game.
"I was not great on returns and was struggling on serve. He completely deserved the victory."
The Austrian was on the back foot early as he lost the opening set to his 52nd-ranked opponent.
Sugita was unable to convert on three match points but hammered his sixth ace to claim victory on a fourth after 88 minutes.
The Japanese will be playing a grass court quarter-final for the second time in his career when he takes on American qualifier Denis Kudla, who defeated Greek teenager Stefanos Tsitsipas 6-3, 6-4.
Against Thiem, who leads the ATP with 36 match wins this season, Sugita kept up the pressure on his rival's serve, forcing the Austrian to save 10 of 13 break points before dropping serve in the last game.
Thiem will return to Austria for five days of rest and fly to London in midweek.
However, he remains unconcerned with the state of his grass game after also losing in Halle in the second round a year ago before reaching a career-best Wimbledon fourth round.
"Last year I played the same as I did this year and played well at Wimbledon. I would have rather done much better," added Thiem.
"But margins are much tighter on grass. If you have a bad day you can't dig it out like I can on clay. If I don't play at 100 per cent -- like today -- the match is lost."
Nishikori has a mixed record at the Wimbledon tune-up event, dominated over the past decade and a half by Roger Federer with nine titles.
The world number 27 Japanese is coming back from wrist injury problems and working to rebuild his ranking, which once placed him firmly in the Top 10.
He began 2018 by playing a lower-level Challenger tournament before returning to the ATP in February.
The 28-year-old reached the Halle semi-finals in 2014 and 2015 but also exited injured in 2016 and a year ago, when he quit against Khachanov trailing 3-2 in the first set.
"I made too many unforced errors, I had a lot of mistakes," Nishikori said.
"He was playing good, but nothing special, I couldn't make a first serve and I should have stayed back more at the baseline.
"Khachanov hits a flat ball and that makes it not so easy on grass. I need to train more before Wimbledon."
Khachanov advanced to a Friday quarter-final against Spanish fourth seed Roberto Bautista Agut, a 6-4, 7-5 winner over Dutchman Robin Haase.
Khachanov managed seven aces against his Japanese opponent, who was broken four times.
Results at the ATP tournament in Halle on Wednesday (x indicates seed):
Nikolaz Basilashvili (GEO) bt Rudolf Molleker (GER) 6-4, 7-6 (7/5)
Yuichi Sugita (JPN) bt Dominic Thiem (AUT x3) 6-2, 7-5
Denis Kudla (USA) bt Stefanos Tsitsipas (GRE) 6-3, 6-4
Roberto Bautista Agut (ESP x4) bt Robin Haase (NED) 6-4, 7-5
Karen Khachanov (RUS) bt Kei Nishikori (JPN x7) 6-2, 6-2