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 number one.
Nadal, who received a bye as second seed, was playing his first singles match in two months after spending time at home in Mallorca with his wife and newborn son.
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.
"I had the match (in my hands) in that second set with a set and break. I played a terrible game there. I didn't deserve the victory playing that bad in that key moment," said Nadal.
Nadal is a 14-time French Open champion but has never won the title at Bercy.
"A lot of things have been going on the last couple of months, without a doubt. But we are always ready to find excuses. At the end, it's always the same. You play well, you win; you don't, you lose," he added.
"If nothing happens, I hope to be there. I'm excited about playing, even if it hasn't been the perfect couple of months for me."
Paul goes on to play Pablo Carreno Busta for a place in the quarter-finals.
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 number one," said Alcaraz.
The Spaniard was presented with the world number one trophy earlier in the week, having become the youngest player to top the ATP rankings in September.
"I felt number one 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."
Nadal's loss means Alcaraz will end the season as the top-ranked player if he takes home the trophy on Sunday.
- Medvedev beaten -
Nadal's early exit was preceded by that of Daniil Medvedev, the 2020 Paris champion and 2021 runner-up 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 semi-finalist 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 tournament's eight-man field 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 meeting with third seed Casper Ruud.