Stuttgart - 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 either Frenchman Gilles Simon or Canadian teenager Felix Auger-Aliassime in his first top-level quarter-final since February 2017.
Results from the ATP tournament in Stuttgart on Thursday (x denotes seeding):
Dustin Brown (GER) bt Alexander Zverev (GER x1) 6-4, 6-7 (3/7), 6-3
Denis Kudla (USA) bt Gaël Monfils (FRA x5) 7-5, 6-7 (3/7), 7-6 (7/3)
Matteo Berrettini (ITA) bt Karen Khachanov (RUS x2) 6-4, 6-2