Bianca Andreescu is the first Canadian to win the Rogers Cup in 50 years.
She was up 3-1 in the first set when American opponent Serena Williams called for a medical timeout.
Less than a minute later, the chair umpire announced that Williams was retiring from the match, handing Andreescu her second WTA Premier title of the season.
The 19-year-old Andreescu has now won 17 consecutive matches, not counting when she has had to retire from matches due to injury herself.
Andreescu, from nearby Mississauga, Ont., now has victories over seven of the top 10 players in the world.
Her world ranking will rise from 27th to 14th on Monday. Her previous high was 22nd.
The Rogers Cup was Andreescu’s first tournament after a right-shoulder injury sidelined her since the French Open in May.
Andreescu had been on the court more than any other player at this year’s Rogers Cup at 10 hours 54 minutes heading into the match against Williams.
The tournament’s final lasted only 16 minutes before Williams withdrew.
After the chair umpire announced Williams’s retirement the former world No. 1 started to cry on her bench. Andreescu went over to comfort her, hugging her and telling Williams how much she admires the 23-time Grand Slam winner.
Faye Urban of Windsor, Ont., beat Vancouver’s Vicki Berner in the 1969 final, when the tournament was still played on clay courts and called the Canadian Open.
“I’m sorry I couldn’t do it today. I tried but I just couldn’t do it,” Williams told the crowd through tears after revealing she was suffering from a back injury. “It’s been a tough year but we’ll keep going.”