Carlos Alcaraz of Spain hits a return to Tommy Paul, of the United States, during the National Bank Open men’s tennis tournament, Aug 11, 2023, in Toronto. (PHOTO / XINHUA)

Carlos Alcaraz's US Open preparations suffered a setback on Friday as the world number one fell to American Tommy Paul 6-3 4-6 6-3 in the quarterfinals of the Canadian Open.

Tommy Paul took control in the decider, smoking a backhand at the charging Alcaraz that he could not put back in play for a break and pouncing on a poorly executed drop shot from Alcaraz on match point to dismiss the top seed

For the third consecutive match, the Spaniard got off to a sluggish start on the hard courts in Toronto, falling behind early and double faulting to hand Paul the first set.

Alcaraz upped his game in the second, hitting a sensational tweener for a winner to level at 3-3 and breaking at love for a 4-3 lead the next game en route to leveling the contest.

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But Paul took control in the decider, smoking a backhand at the charging Alcaraz that he could not put back in play for a break and pouncing on a poorly executed drop shot from Alcaraz on match point to dismiss the top seed.

The 26-year-old Paul will rise to a career-high ranking of at least 12 on Monday and will face either Gael Monfils or Jannik Sinner in the semis.

Earlier, Alex de Minaur pulled off a gritty 7-6(7) 7-5 upset of second seed Daniil Medvedev at the Canadian Open on Friday to reach his first semi-final at a Masters 1000 event where he will face Spaniard Alejandro Davidovich Fokina.

The speedy Australian fell 5-2 behind in the opener and had to fend off three set points in the tiebreak before clinching the first set against the 2021 champion in Toronto.

The 24-year-old De Minaur had to rally from a break down twice in the second set before showing his resilience again thanks to his brilliant return game, sealing victory on the Russian's seventh double fault.

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"I'm probably most proud of my mental state," de Minaur told reporters.

"Throughout the whole match, I thought I was very level-headed and at no point in that match did I feel frazzled at all," he said.

"I knew my game plan. I stuck to it. I stayed positive. And I knew that any small chances I was going to get, I was going to have to take them and try to be aggressive and, you know, it worked out. So extremely happy with that."

Up next is a meeting with red-hot Spaniard Alejandro Davidovich Fokina, who swept aside American Mackenzie McDonald 6-4 6-2 earlier on Friday.