British veteran Andy Murray withdrew from his last 16 clash with Italy's Jannik Sinner at the ATP Toronto Masters on Thursday, citing an abdominal strain.

The 36-year-old two-time Wimbledon champion, who had battled into the third round after a gruelling three-setter against Max Purcell on Wednesday, announced his withdrawal moments before his clash with Sinner was due to get under way.

"Everyone I have an issue with my abdominal, so unfortunately I'm not going to be able to play this evening," Murray told fans on court in Toronto.

"I'm really sorry. I feel like I've let you down. I've rarely been in this situation in my career and I feel terrible."

Murray's withdrawal gave seventh-seed Sinner a walkover into a quarter-final against French veteran Gael Monfils.