Top seed Rafael Nadal suffered a shock straight-sets defeat to Dustin Brown in his first match on grass this year at the Gerry Weber Open in 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 in the Halle second round.
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.
"I didn't play," Nadal said. "That can happen when you play an opponent like this. But I don't want to talk about the opponent.
"For the first four games he didn't put any returns on the court and then suddenly he was playing winners from the return, so it's difficult to analyze these kinds of matches.
"The only thing I can do is keep working the way that I did the last three days, because I came here, I tried hard, I tried my best."
Brown, from Germany, said: "I never played against a world number one before, never played him before, so this is just crazy for me."
Federer, a six-time champion in Halle, was playing his first match since he lost to Ernests Gulbis in the French Open fourth round.
"The first day when I came here to practise it was bit so-so and everyday after that was very good," said Federer, who will play Taiwan's Lu Yen-hsun in the next round.
"It was important to stay calm (after losing the first set) and actually it gives me more confidence winning this way than maybe just 6-4 6-4, and you don't quite know what is going on."
Third seed Milos Raonic of Canada lost 6-4 6-4 to German wildcard Peter Gojowczyk on Wednesday.