Andy Murray apologised to the crowd at the Canadian Open after withdrawing from the tournament with an abdominal strain.
The 36-year-old Briton was due to face Italian seventh seed Jannik Sinner in the last 16 after beating Max Purcell in the second round.
Addressing the fans in Toronto, he said: “I feel like I’ve let you down.”
Murray added: “This might be my last time playing here as well. So to finish like this feels rubbish.”
The Canadian Open alternates between Toronto and Montreal, so the men’s event will be staged in the latter next year.
This is only Murray’s seventh time conceding a walkover in his career. Sinner advances to the quarter-finals and will face Frenchman Gael Monfils on Friday.
“It took me about 10 to 12 days before I was feeling good again,” the three-time Grand slam winner added.
“This is not as bad but obviously the danger if you compete and play on it is you make it worse.
“So, I’ll need to see how it develops over the coming days and hopefully feel better in a few days. I’m really sorry. Thank you.”
The abdominal strain will disrupt Murray’s preparations for the final Grand Slam of the season at the US Open, which begins 28 August in New York.
Meanwhile, world number one Carlos Alcaraz is through to to the quarter-finals after a tough battle with 15th seed Hubert Hurkacz.
The Wimbledon champion won 3-6 7-6 (7-2) 7-6 (7-3). He led 5-2 in the deciding set before squandering match points to hand Poland’s Hurkacz a lifeline.
“Honestly, I didn’t know what happened,” said the 20-year-old. “I started feeling bad in that moment.
“The big players have that feeling to find a way to stay alive and try to end the match playing well.”
Alcaraz, who won his maiden Grand Slam title at the US Open in 2022, extended his winning streak to 14 matches and will play American 12th seed Tommy Paul next.