Ajla Tomljanovic lasted just 22 minutes in her second round clash with Pole Iga Swiatek before withdrawing.
Tomljanovic was losing 4-1 in the first set when she retired from the match.
While the extent of the injury or discomfort is yet to be confirmed, the retirement is a poorly-timed scupper in an otherwise great few months of form for Tomljanovic.
The Australian baseliner hit a career high ranking of 39 in April, however may drop further below her current 47 after the withdrawal.
She's coming off a second round loss at Wimbledon to Barty's doubles partner Victoria Azarenka and a Round of 32 loss at the Silicon Valley Open to Kristie Ahn.
The performance summed up a horrible day for the Aussies in Canada, with world-number one Ash Barty set to lose her spot at the pinnacle of women's tennis if Naomi Osaka wins her match tomorrow.
After a disappointing Wimbledon campaign, Barty suffered a three-set second round loss to Sofia Kenin, while John Millman was the only Australian winner in the men's draw.