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.
On the other hand, second seed Karen Khachanov was also upset by Matteo Berrettini 4-6, 2-6. Earlier this week, Berrettini also registered a straight sets win over Nick Kyrgios.
It was the second serve that cost Khachanov as he registered only 44 per cent points on them compared to Berrettini getting 71 per cent of those.
With the loss of Zverev and Khachanov, Stuttgart Open has lost both its top seeds before the quarter-final stage.
Fifth seed Gael Monfils also bowed out of the tournament after a 5-7, 7-6(3), 6-7(3) loss to Denis Kudla.
Canadian Felix Auger Aliassime was the only seed to advance on the day as he beat Gilles Simon 7-5, 6-4.
(With AFP inputs)