World number two Carlos Alcaraz was knocked out of the Shanghai Masters in straight sets by 33rd-ranked Tomas Machac on Thursday, losing 7-6 (7/5), 7-5.
The Czech will face world number one Jannik Sinner in the semi-finals, after a surprisingly straightforward 6-1, 6-4 victory over an injured Daniil Medvedev earlier.
Alcaraz's match came shortly after his idol and compatriot Rafael Nadal announced he would retire after the Davis Cup finals in November.
"Honestly I couldn't believe it," said Alcaraz, but insisted it hadn't affected his playing.
"Thankfully I saw it one hour before the match, so I had time to accept it and forget it."
The 21-year-old told reporters that to see Nadal "leave tennis, which is what he loves, is painful", adding it was "difficult news for everyone".
Sinner for his part called the Spaniard an "unbelievable person" as he spoke about the positive impact Nadal had on young players like himself.
"It's tough news for all the tennis world and not only (the tennis world)," he said.
- 'Unbelievable' -
The bad news continued for Alcaraz as he was stunned by 23-year-old Machac's hard-hitting pace.
"His level was so high, I thought he was going to give me a window of opportunity, but he didn't... It was unbelievable. It was crazy for me," the four-time Grand Slam champion said afterwards.
The Czech edged him out in a closely fought first-set tiebreak after neither player was able to break their opponent's serve, AFP reported.
The Spaniard looked like he might make a comeback when he broke in the sixth game of the second set to level.
However, Machac powered through and broke Alcaraz again in the 11th game to claim the shock victory and progress to the semi-finals.
"I knew that against Alcaraz I have to play at this level to have a chance to win," said Machac, saying there were "no other options".
He said he would try more of the same against Sinner.
"He's playing unbelievable tennis... but I'm really looking forward to the challenge, so I will enjoy it," Machac said.
- Sinner takes advantage -
Sinner, who Alcaraz beat in the China Open final last week, looked strong from the beginning of his match against Medvedev.
This was the fifth time the two have met in the latter stages of tournaments this year, and the Italian has won four of those clashes.
In the first set, which lasted only 25 minutes, the Italian broke Medvedev in the second and sixth games to murmurs of surprise from the crowd.
The Russian kept holding his shoulder, which he had said the day before had "some niggles", and received medical attention several times during the match.
"The energy of a winner, I would say today I didn't have it unfortunately because of my physical condition," said Medvedev.
"And that's going to be the focus to try to get it back."
"He didn't play at his best," said Sinner, acknowledging the injury.
"But this can happen and I took advantage of that today. I felt like I was playing some good tennis, especially the first set, trying to keep going in the second set. It was a good performance from my side."
