Zizou Bergs defeated Carlos Alcaraz in straight sets at the Shanghai Masters on Monday, halting the former world No. 1's successful return from a monthslong injury layoff.
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 either Tommy Paul or Valentin Vacherot next.
"It's a little disbelief and not feeling too much," Bergs, 27, said afterward.
"You're just really in the moment, being focused on the points you have to play," he added.
"I'll be very happy."
Alcaraz started with more quality until Bergs broke the Spaniard's serve to go up 3-2 in the first set.
The 23-year-old 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 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.
The Spaniard forced a tie-break but could not reach a third set as Bergs closed things out just past the two-hour mark on his first match point.
Fritz fumbles
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 afterward.
"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.