Carlos Alcaraz was unbeaten in 14 matches before his defeat by world number 14 Tommy Paul on FridayWorld 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.
"I realise that I didn't play well, these matches," said Alcaraz of his week in Toronto. "All I can do now is practise to be better. I have some weeks before the US Open.
"I think I did well some things, but probably everything can be better. I always say you can be better in everything even if you've played your best matches in your career.
"But right now, I have to improve a lot of things, getting more confidence in my game."
Alcaraz said he will now focus on the Western and Southern Open in Cincinnati, where he will be top seed, before beginning the defence of his US Open title in New York on 28 August.
World number three Medvedev had a 4-0 lead in the opening set tie-break before 24-year-old De Minaur fought back.
In a tight second set, Medvedev was 6-5 down and serving to set up another tie-break but was broken with De Minaur sealing victory on his second match point.
"I had to play some of my best tennis today. It has been a great week for me so far. I played the right way today," said De Minaur.
"I think having played him at the end of last year gave me the confidence that I had a chance."
