Rafael Nadal said his shock quarter-final defeat to Alexander Zverev at the Madrid Open was "very difficult to understand". 

Nadal, a five-time champion at the event, was a break up in the first set of his 15th quarter-final at Caja Magica but lost 6-4 6-4 in an hour and 44 minutes.

The win was the biggest of Zverev's career on clay and confirmed his rise to second in the ATP Rankings. 

While Nadal said he was largely pleased with his displays in the Spanish capital, he was at a loss to explain the manner of his defeat on Friday. 

"I'm leaving Madrid with an overall positive feeling, but at the same time with the ugly feeling of having played a match like this today against a great player," Nadal told a media conference. 

"I think for most of the first set, I was playing better than him. And this is the negative part; while playing better than him in the first set, I still lost 6-4.

"This is very difficult to understand, especially for me. When this happens to me, it's usually in the opposite way. I find a way to win sets even though I'm not playing my best or as good as the opponent.

"I felt like I was playing better for much of the first set, but after a couple of errors – unjustifiable errors at the worst times – I found myself down a set.

"The outlook of the match changed there, both for me and for him. I knew then that a lot of suffering was waiting for me [in the second set], and for him, the knowledge that he just took a huge step forward during the match."

Zverev will face Dominic Thiem in the last four after the Austrian overcame John Isner 3-6 6-3 6-4 in an hour and 55 minutes. 

Thiem, who has now made it to the past four Madrid Open semi-finals, said: "I think we all know that he is one of the best servers in history and the altitude here in Madrid makes it even tougher to return his serve.

"I was a little bit surprised by his return games. I think he actually attacked both of my serves, the first and the second, and it took me a while to get used to it."

Casper Ruud will meet Matteo Berrettini in the other semi-final. 

Norwegian Ruud eased past Alexander Bublik 7-5 6-1 to make it three consecutive clay ATP Masters 1000 semis, while Berrettini beat Cristian Garin 5-7 6-3 6-0. 

Rafael Nadal saw off Alexei Popyrin to reach the Madrid Open quarter-finals on a day that saw Daniil Medvedev and Stefanos Tsitsipas suffer surprise defeats.

World number two Nadal, chasing a record-extending sixth title on home soil, advanced 6-3 6-3 in a time of one hour and two minutes on Thursday.

Popyrin, playing in his first last-16 match at a Masters 1000 event, broke Nadal in the sixth game of the opening set but the Spaniard otherwise looked comfortable.

Nadal converted four of his six break points to advance through to the last eight for a 15th time, where Alexander Zverev awaits.

"It was difficult," Nadal said in his on-court interview. "The court today was slippery, it was very fast, very dry conditions. The ball was flying a lot. The beginning was super tough. 

"He was hitting every ball and hitting the spots. I'm happy with the victory. It was an important victory, he came here playing well. Every match is tough."

Zverev, who won the competition in 2018, beat British number one Dan Evans 6-3 7-6 (7-3) to set up a meeting with Nadal

However, second and fourth seeds Medvedev and Tsitsipas saw their participation come to an end at the hands of Cristian Garin and Casper Ruud respectively.

Medvedev, who is still yet to reach the quarters in Madrid, lost the only break of serve in the third game of the opening set but recovered by taking the second set in a tie-break.

Garin dominated the deciding set, the world number 25 breaking Medvedev twice and, after a short delay to fix a broken net, advanced 6-4 6-7 (7-2) 6-1.

Tsitsipas, who held championship point against Nadal in the Barcelona Open final, failed to break Ruud on a single occasion as he fell 7-6 (7-4) 6-4.

Alexander Bublik is next up for Ruud after beating Aslan Karatsev 6-4 6-3, while Matteo Berrettini - 7-6 (7-4) 6-4 winner against Federico Delbonis - will meet Garin.

Also vying for a place in the semi-finals are John Isner and Dominic Thiem, who are set to face off after beating sixth seed Andrey Rublev and Alex de Minaur respectively.

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