Marin Cilic crashed out of the Open Sud de France in the second round as he was stunned by German teenager Alexander Zverev in Montpellier on Wednesday.
Second seed and former US Open champion Cilic, who lost in the third round of the Australian Open, suffered a 7-6 (7-4) 7-6 (7-1) defeat to the 18-year-old.
Zverev came from a break down in both sets and dominated each of the tie-breaks to set up a quarter-final clash with compatriot Michael Berrer, who defeated Kenny De Schepper 6-3 6-4.
"It's one of the biggest wins of my young career yet," Zverev said. "I'm very happy to be in the quarter-finals. Just a wonderful match."
Cilic was not the only seed to succumb to an early exit, Gael Monfils beaten by fellow Frenchman Edouard Roger-Vasselin in the day's only first-round match.
Fifth seed Monfils - a quarter-finalist in Melbourne - fired down 10 aces - but was unable to prevent Roger-Vasselin from claiming a 6-4 7-6 (7-5) success.
There were no such problems for top seed Richard Gasquet, who dropped just four points on his first serve en route to a 6-4 6-4 win over Ernests Gulbis.
Gasquet will meet eighth seed Marcos Baghdatis in the quarters after the Cypriot saw off Taro Daniel 6-2 7-6 (9-7).