Carlos Alcaraz fell to Zizou Bergs in straight sets at the Shanghai Masters on Monday, interrupting the former world number one's in-form return to the professional Tour after being sidelined for months by injury.
The 39th-ranked Belgian beat the second-seeded Spanish star 7-5, 7-6 (7/4) in the third round to land the tournament's biggest upset yet, and will play Tommy Paul next.
"Today wasn't a good match from my side. I didn't feel good," Alcaraz told reporters, crediting Bergs for taking chances and serving big.
"In a few important moments I didn't play my best, and I would say there were a few moments that he was lucky," the 23-year-old said of their two-hour battle.
Alcaraz said he was "disappointed" by a loss riddled with errors, which came a week after he won the Tokyo Open, adding he would return home and hit practice.
"Right now, you know, all the negative thoughts come to my mind and none of the positives, but I think I will take it, you know, the good things," he said.
His loss also ensures current world number two Alexander Zverev, the reigning French Open and US Open champion, will replace Jannik Sinner as world number one by the year's end, with the Italian's season over due to a knee injury.
Zverev, the top seeded player in Shanghai, will face Alexander Bublik in the last 16 on Tuesday.
Alcaraz started with more quality until Bergs broke the Spaniard's serve to go up 3-2 in the first set.
Alcaraz grabbed back that break in the next game but was uncharacteristically off-target, sending a mess of shots into the net.
He double-faulted to hand Bergs a critical break point in the 11th game, and lost the first set when a volley went short.
Alcaraz, currently ranked third in the world, was visibly frustrated during the second set as he vented to his corner following a series of misfired shots that left him with no points in the fourth and eighth games.
He forced a tie-break but could not reach a third set as Bergs closed things out on the first match point.
"It's a little disbelief and not feeling too much," Bergs, 27, said after winning.
"You're just really in the moment, being focused on the points you have to play," he said.
Earlier, Stefanos Tsitsipas came back to beat Taylor Fritz 3-6, 7-5, 6-3 and earn fourth round meeting with Daniil Medvedev, who brushed aside qualifier Dalibor Svrcina 6-2, 6-3.
A wardrobe malfunction delayed victory for the Greek world number 43 when a tennis ball tumbled out of the American's pocket a second time and forced the men to replay a match point.
A tired-looking Fritz, seeded 11th, hung on for a bit longer to save the eighth game but could not stick around beyond that, with Tsitsipas holding serve to win at the fifth time of asking.
"I could have done a few things easier, but I ended up being my biggest enemy," Tsitsipas told the crowd afterwards.
"I turned things around really well towards the end and became friends with myself and played some incredible tennis," he said.
Felix Auger-Aliassime, Casper Ruud and Rafael Jodar also went through to the last 16.
American world number 18 Paul sent off last year's winner Valentin Vacherot 6-4, 6-2 while Botic van de Zandschulp beat eighth-seeded Arthur Fils 7-5, 6-4.
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