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French Open champion Zverev appeared to struggle with the hot conditions in Germany, with Fritz taking advantage to dump the home favourite out.

Fritz, who reached the Stuttgart Open final before losing to Ben Shelton last week, was on the back foot after being broken in game three of the opening set, but he struck back to love in game six, and a tie-break was then required.

It was Zverev that came through a tight breaker, but the heat then started to become a factor as he repeatedly dropped to his haunches between points in the second set.

Zverev desperately attempted to keep the points short where possible, but a single break in game nine – again to love – saw Fritz take the second set.

The players left the court before the decider, and the break seemed to benefit them as they both served imperiously, at least until the penultimate game.

With the set finely poised at 5-5, Zverev produced another error-strewn service game, with Fritz converting his third break point before serving the contest out to tee up a final clash with countryman Frances Tiafoe, who beat Daniel Altmaier 6-1 6-3 in te other semi-final.

"I felt like I played a good tiebreak [in the first set] and I felt I got a little unlucky with some bounce and he played some really good points and took one on my serve and that was it," Fritz said.

"I felt like he was struggling with something. I am not sure what it was. I felt like I felt better than he did so that was what I needed to tell myself to go to work."