Two-time French Open quarter-finalist Alexander Zverev suffered a shock defeat in the second round of the Stuttgart Open to compatriot and world number 170 Dustin Brown.
German Zverev lost 6-4 6-7 (3-7) 6-3 in the first grass-court tournament of the season before Wimbledon.
The world number five, who is yet to win a title on grass, led 5-2 in the second but was taken to a tie-break.
He missed three break points at 3-3 in the decider before Brown broke late.
Brown, 34, who famously knocked out Rafael Nadal in the second round of Wimbledon in 2015, saved 13 of 14 breakpoints as he claimed only his fifth win over a top-10 ranked player.
Elsewhere, Greek top seed Stefanos Tsitsipas lost 6-4 3-6 6-4 to Chilean world number 60 Nicolas Jarry in the first round in 's-Hertogenbosch.
The world number six was playing in his first grass-court match since a dramatic fourth-round French Open exit against Swiss Stan Wawrinka in Paris.
Belgian fifth seed David Goffin comfortably beat Frenchman Pierre-Hugues Herbert 6-3 7-5 in just over an hour and a half in the Netherlands.
Jamie Murray and Neal Skupski's new partnership suffered its first defeat as the third seeds were beaten 7-5 6-3 by Robin Haase and Jean-Julien Rojer in the doubles quarter-finals.
The British duo came together after Murray called time on his three-year association with Brazilian Bruno Soares.