French Open finalist Thiem never really got going against the world number 52, who won the first four games en route to a 6-2 7-5 triumph.

Sugita took his time to break the Austrian in the second set and then required four match points to clinch victory, but he got the job done to earn a huge scalp.

"It's unbelievable. For sure, this is the best match in my career," Sugita said. "I'm really happy to win."

Seventh seed Kei Nishikori was also on the end of an upset as Karen Khachanov continued his fine year, the Japanese beaten comprehensively 6-2 6-2.

Russia's Khachanov broke early in both sets to set up a last-eight meeting with Roberto Bautista Agut - a rare winning favourite - on Friday.

Bautista Agut battled past Robin Haase 6-4 7-5, while Denis Kudla also progressed and Nikoloz Basilashvili beat Rudolf Molleker in the round of 32.