Rafael Nadal crashed out of the Paris Masters in the second round on Wednesday after losing in three sets to American Tommy Paul, boosting Carlos Alcaraz's hopes of finishing the year as world No 1.
Nadal was playing his first singles match in two months after a short break at home in Mallorca with his wife and newborn son.
The 22-time Grand Slam champion shook off an early break in the opening set by winning five of the next six games.
But Paul recovered from dropping serve in the third game of the second set to win a tie-break, having failed to convert a set point with Nadal serving at 4-5.
The Spaniard's lack of match practice told as Paul dominated the final set, breaking three times to seal the biggest win of his career.
His reward is a meeting with Pablo Carreno Busta for a place in the quarterfinals.
Alcaraz, the reigning US Open champion, will play Grigor Dimitrov in the last 16 after defeating Japan's Yoshihito Nishioka 6-4, 6-4.
The 19-year-old smacked 30 winners including nine aces despite a heavily-taped left knee, an issue he had downplayed on Monday.
"I tried to find my best level. I think I played really well. I don't think about the pressure of being world No 1," said Alcaraz.
The Spaniard was presented with the world No 1 trophy earlier in the week, having become the youngest player to top the ATP rankings in September.
"I felt No 1 before getting the trophy, but of course with the trophy, is amazing. It was amazing feeling when I had the trophy in my hands."
Alcaraz admitted there is a target on his back now as the world's top-ranked player.
"Probably the difference, the way the players play against me, probably I felt that difference, but not too much," he said.
"I would say the players play better or more aggressive with me."
MEDVEDEV BEATEN
Nadal's early exit was preceded by that of Daniil Medvedev, the 2020 Paris champion who was knocked out by Australia's Alex de Minaur 6-4, 2-6, 7-5.
De Minaur, who had not previously beaten a top-five player, will now face US Open semifinalist Frances Tiafoe, who eased past Jack Draper 6-3, 7-5.
Felix Auger-Aliassime qualified for the season-ending Tour finals for the first time, with Andrey Rublev completing the eight-man field in Turin after Taylor Fritz and Hubert Hurkacz both lost.
Auger-Aliassime began his bid for a fourth title in four weeks by battling back from the brink to beat Mikael Ymer in three hours and 30 minutes.
The Canadian eighth seed scraped through 6-7 (6/8), 6-4, 7-6 (8/6) to extend his winning streak to 14 matches.
Auger-Aliassime saved two break points at 1-4 in the second set just as Ymer looked poised to end his unbeaten run.
"Somehow I found a second wind after saving those break points at 4-1," said Auger-Aliassime.
"It was pretty epic. Definitely a win to remember."
Fritz was upset 7-5, 5-7, 6-4 by the 37-year-old Gilles Simon, who is retiring after the tournament.
Hurkacz's qualification hopes ended with his 7-5, 6-1 loss to Danish teenager Holger Rune.
Fifth-ranked Stefanos Tsitsipas had no problem in seeing off Britain's Dan Evans 6-3, 6-4 to book his place in the last 16.
Italy's Lorenzo Musetti beat Nikoloz Basilashvili of Georgia 6-4, 6-2 to set up a last-16 meeting with third seed Casper Ruud.