Frenchman Gael Monfils withdrew hours before his match against Novak Djokovic in the Paris Masters on Thursday due to an adductor strain, sending the Serbian world number one into the quarter-finals.
After his second round win over compatriot Adrian Mannarino, Monfils said he got injured during the match and an ultrasound on Thursday revealed a muscular lesion.
"I have the impression that to be optimistic, in about 10 days I will be at 100%, but because of muscular lesion, you just need rest," Monfils told reporters, confirming that it was the end to his 2021 season.
Djokovic, playing his first tournament since finishing runner-up in the U.S. Open, will take on American Taylor Fritz in the last eight.
World number two Daniil Medvedev needed three sets to see off Sebastian Korda of the United States 4-6 6-1 6-3. The US Open Champions will face next face Hugo Gaston after the Frenchman's 6-4, 7-5 win over Carlos Alcaraz of Spain.
Casper Ruud crushed American Marcos Giron 6-2, 6-1 and became the first ever Norwegian to qualify for the ATP Finals. Ruud will meet German Alexander Zverev in the next round after the latter edged Bulgarian Grigor Dimitrov 7-6, 6-7, 6-3.
"It's been an unbelievable year that will have the perfect end to the season," Ruud said in an ATP statement after confirming his Turin spot.
"It's been a stressful couple of weeks and months, because it's been such a tight race. It's been fun at the same time (and) so exciting to watch as a tennis fan."
Poland's Hubert Hurkacz also booked his place in the quarter-finals with a 4-6, 7-5, 6-2 win over German Dominik Koepfer, to improve his chances of qualifying for the Nov. 14-21 ATP Finals in Turin.
The season-ending tournament is played between the top eight men's singles players and doubles teams.
Djokovic, Stefanos Tsitsipas, US Open champion Daniil Medvedev, Alexander Zverev, Andrey Rublev and Matteo Berrettini have qualified for the event with two spots yet to be confirmed.
Hurkacz currently sits in the final automatic qualification spot and could qualify by beating American James Duckworth in the quarter finals.