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American Shelton beat Jiri Lehecka 6-4 6-4 and will take a spot in the top 10 of the world rankings after countryman Reilly Opelka did him a favour at the Libema Open in‘s-Hertogenbosch, Netherlands, upsetting No.1 seed Daniil Medvedev.

"That is really cool," Shelton said after being told he'd break into the world's top 10.

"A big milestone and in tennis. I guess Stuttgart is the place for Americans to break into the top 10 because you all saw Frances Tiafoe do it a couple of years ago when he won that title.

"I am just thinking about this week, I want to keep going this week and want to be playing on Sunday [Monday AEST]."

Shelton produced 18 aces in his victory over Lehecka, winning 100 per cent of his first-serve points.

He also did not face a break point against the Czech, and will be fired up to beat home favourite Zverev in the last four.

No.1 seed Zverev claimed one American scalp by defeating Brandon Nakashima 7-5 6-4 in their last-eight clash.

The German saved both break points he faced and hit 31 winners past Nakashima as he continues his quest for a maiden grass-court title on the ATP Tour..

Zverev has already lifted a trophy in Germany, claiming the Munich Open title on clay earlier this year.