Aryna Sabalenka saved three match points to stun Caroline Wozniacki 5-7 6-2 7-6 (7-4) in the world number two's first match since Wimbledon in the Rogers Cup second round.
The Dane's last outing had seen her tumble out in the second round at the All England Club, suffering a surprise defeat to Ekaterina Makarova.
She subsequently took the precautionary measure of pulling out of the WTA Citi Open due to muscle issues and, playing with strapping on her right thigh, was on the receiving end of another shock.
Wozniacki appeared poised for victory after going a break up in the third set, but Sabalenka, who hit 64 winners and 54 unforced errors, produced a turnaround to prevail on Thursday.
Second seed Wozniacki won four straight games to come from 5-3 down to take the first set, leading an impassioned Sabalenka to drop her racquet in disgust.
But the Belarusian channelled that frustration magnificently in the second, which she clinched in 31 minutes, breaking in the fourth game and then dominating the rally on set point, taking it with an emphatic smash.
Australian Open champion Wozniacki appeared in command when an errant cross-court pass from Sabalenka handed her break in the third. However, the former could not capitalise on any of the match points on the Sabalenka serve and then handed the break back with a double fault.
The power that had been prevalent throughout Sabalenka's performance came to the fore in the tie-break, and she confidently dispatched a forehand to set up a third-round clash with Elise Mertens.