Raonic slightly tore his right hamstring Saturday night, feeling a sharp pain while chasing down a drop shot in his semifinal victory over Juan Martin del Potro.
Raonic tried to ignore the discomfort, opting to go to sleep and hoping he would feel fine in the morning. Instead, after getting two hours of treatment, the 26-year-old Canadian ranked fourth in the world hobbled onto a practice court where Sock was warming up and announced he would be unable to play.
The third-seeded Sock wound up with his second title of the year and moved up to a career-best 18th in the ATP rankings. The 24-year-old also won last month in New Zealand and helped the U.S. reach the quarterfinals of the Davis Cup.
Sock is 11-1 this year, with his only loss to Jo-Wilfried Tsonga in the third round of the Australian Open.
Only Roger Federer, who won the Australian Open, has a better winning percentage that Sock this year. He also helped lead the U.S. to the Davis Cup quarterfinals with a first-round win over Switzerland.
"This is disappointing in a lot of ways," said Raonic, who also suffered an adductor injury during his run to the Australian Open quarterfinals that forced him to pull out of Canada's Davis Cup tie versus Great Britain. "I've been unfortunate with suffering injuries in three tournaments in a row. Being one match away from a title and not being able to compete, that's not easy to accept."
Sock, who supplanted John Isner as the top-ranked American male last October, said the walkover was "not ideal for either side, but a title is a title."
"I've been playing well. Now I just have to keep my head down and do everything I can to progress," he said.
In the doubles final, the second-seeded team of South African Raven Klaasen and American Rajeev Ram defeated third-seeded Treat Huey of the Philippines and Belarusian Max Mirnyi 6-3, 3-6 and 10-1 in the super tiebreaker. It was the fourth ATP title for the duo dating to 2015.