World number two Alexander Zverev suffered a shock early exit from the Mexican Open with a straight-set defeat against teenage qualifier Learner Tien.
Australian Open finalist Zverev, the top seed in Acapulco, put in an uncharacteristically error-strewn performance as American Tien won 6-3 6-4.
Tien, 19, beat Briton Cameron Norrie in the previous round and will face Czech Tomas Machac in the quarter-finals.
"I just went out there and tried to control my side of the court," said Tien.
"Obviously I knew it was going to be a tough match. To come through feels great.