Top seed Rafael Nadal suffered a shock straight-sets defeat to Dustin Brown in his first match on grass in 2014 at the Gerry Weber Open in Halle, Germany.
The Spaniard, who won his ninth French Open title on Sunday, lost his serve three times as he lost 6-4 6-1 in under an hour.
World number 85 Brown will play Philipp Kohlschreiber in the quarter-finals.
Roger Federer began his preparations for Wimbledon with a three-set victory over Portugal’s Joao Sousa.
The Swiss defending champion and second seed won 6-7 (8-10) 6-4 6-2 to reach the last eight.
It was Federer’s first match since he lost to Ernests Gulbis in the French Open fourth round.
Federer, a six-time champion in Halle, will play Taiwan’s Lu Yen-hsun in the next round.
“The first day when I came here to practice it was bit so-so and everyday after that was very good,” said Federer.