Stan Wawrinka hit 41 unforced errors on Tuesday as he crashed out of the first round of the ABN AMRO World Tennis Tournament, losing 4-6 6-3 6-2 to 259th-ranked Tallon Griekspoor.
It was the first win in an ATP tournament for the 21-year-old Dutchman, who dominated the second and third sets against a lacklustre Wawrinka.
"Unbelievable, I have no words for it, beating such a great champion," Griekspoor said.
Wawrinka was downbeat as expected.
"It was a tough day at the office," Wawrinka said. "I still need a lot of work to get back.
"There are a lot of ups and downs. Today was not my best day. He played well and went for it. He played great and deserved to win."
Earlier, fourth-seeded David Goffin took just more than an hour to beat Nicolas Mahut 6-1 6-3.
Goffin heard earlier in the day that his planned opponent at the Ahoy convention centre, Benoit Paire, had pulled out of the tournament with a back injury, opening the door for lucky loser Mahut.
But the Frenchman's stay in Rotterdam did not last much longer as seventh-ranked Goffin conceded just four games.
On Wednesday, 20-time Grand Slam winner Roger Federer begins his campaign in Rotterdam as he bids to return to the top of the world rankings for the first time since 2012.
Federer plays Belgian Ruben Bemelmans in the first round. If the Swiss reaches the semi-finals, he will replace Rafael Nadal atop the rankings.