World number five Alexander Zverev made a poor start to the grass-court season as he was dumped out of the Stuttgart ATP event by fellow German Dustin Brown on Thursday.
The top seed, who lost to Novak Djokovic in the French Open quarter-finals last week, was beaten 6-4, 6-7 (3/7), 6-3 by the 170th-ranked Brown.
The 34-year-old Brown, who famously knocked out Rafael Nadal in the second round of Wimbledon four years ago, saved 13 of 14 break points as he claimed only his fifth-ever win over a top-10 player.
Zverev, 22, has never won an ATP title on grass, while Brown will face Canadian teenager Felix Auger-Aliassime in his first top-level quarter-final since February 2017.
"I am very, very happy," Brown told www.atptour.com. "I didn't have a good grass-court swing last year and just wanted to come and play very good tennis. Stuttgart is a little bit at altitude and I think that helped my game.
"I've won three very good matches, two in qualifying and one against John Millman. The court played a little slower today and it was tough especially against Alex, who is a very good returner.
"So I had to make a lot of first serves and the break in the first set helped me to gain confidence."
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