World number one Carlos Alcaraz was beaten by Tommy Paul in the Canadian Open quarter-finals in Toronto.
Spaniard Alcaraz, 20, made a sluggish start and eventually lost 3-6 6-4 3-6 to American 12th seed Paul.
In his first defeat since the French Open semi-finals in June, Wimbledon champion Alcaraz made 13 unforced errors and served six double faults.
Also on Friday, unseeded Alex de Minaur of Australia caused another upset by knocking out Daniil Medvedev.
De Minaur fought hard to overcome the Russian second seed 7-6 (9-7) 7-5 and reach the semi-finals.
Paul will take on Italian seventh seed Jannik Sinner for a place in Sunday's final, while De Minaur faces unseeded Spaniard Alejandro Davidovich Fokina.