Felix Auger-Aliassime's Shanghai Masters campaign came to an abrupt halt on Wednesday as a knee issue forced the fourth seed to retire against Casper Ruud.

The Canadian was trailing 4-1 in the opening set of his round-of-16 meeting with the Norwegian when he took a medical timeout to receive treatment on his left knee.

Auger-Aliassime appeared to sustain the injury at deuce in game four, when he gave up the first break of the encounter. 

Though he initially attempted to play on, Auger-Aliassime was forced to call time on the contest at 5-2 down after 35 minutes, with Ruud wishing him a speedy recovery.

"Honestly, there's not that much to say. It was very unfortunate for him. It seemed to me that he maybe twisted his knee in the fourth game of the match," Ruud said.

"He came to the net, and I didn't see it 100% because I was focusing on the ball, but it looked like he kind of stumbled a bit with a wrong step. From that moment, it seemed like he was in a lot of pain, which is very unfortunate.

"I'm very sorry for him, because it's a big match for both of us. I know what kind of player he is, what kind of person he is, a very nice person, and you don't wish this on anybody. I hope he has a speedy recovery."

Ruud will now face Rafael Jodar for a place in the semi-finals, after the Spaniard came from a set down to beat Botic van de Zandschulp earlier on Wednesday. 

Daniil Medvedev also booked his place in the last eight with a 6-4 7-5 victory over Stefanos Tsitsipas – it will be the Russian's 28th career ATP Masters 1000 quarter-final and his third of 2026.

The full set is complete From Rome to Shanghai, @CasperRuud98 has reached the quarter-finals at all nine ATP Masters 1000 events. @SH_RolexMasters | #RolexShanghaiMasters https://t.co/qy1SfIY6F9

— ATP Tour (@atptour) October 14, 2026

Auger-Aliassime suffers injury setback

Wednesday's encounter did not end the way anybody wanted, and Auger-Aliassime's participation in the final few events of the season may now be in question.

He had been expected to defend his European Open crown in Brussels next week, ahead of the Swiss Basel Indoors and the Paris Masters – missing those events would spell the end of his hopes of qualifying for the ATP Finals, given he is 15th in the Race to Turin rankings.

Auger-Aliassime's previous meeting with Ruud, at Monte-Carlo earlier this year, had also ended prematurely, with the Norwegian retiring hurt on that occasion.

Ruud is now into his 17th career quarter-final at ATP Masters 1000 events, and his first on hard court since Indian Wells in 2024. 

He will have played a last-eight match at all nine ATP Masters 1000 events, becoming the first player to complete the set since Auger-Aliassime at Monte-Carlo this year.