Johanna Konta appeared on course to win her second-round clash with Victoria Azarenka at the Rogers Cup when rain halted play.
The British number one was 6-3 3-0 ahead when the heavens opened over Montreal on Wednesday evening, forcing matches to be suspended until Thursday.
Rain has been a motif of the Rogers Cup so far, and the delay could leave the Briton with a gruelling day ahead.
If Konta - number 43 in the world - progresses, she is expected to play her third-round tie against world number five Elina Svitolina later on Thursday.
Having battled past number 11 seed Jelena Ostapenko in round one, Konta pulled ahead of former world number one Azarenka in the seventh game of the opening set.
Wild card Azarenka, 29, had served well with two aces, but her younger opponent appeared to have the edge, winning three-quarters of her points on the first serve.
Konta, 27, eventually got the better of the Belorussian, who is coming back after having a son, with a break in the seventh game.
After two attempts in the ninth, she broke Azarenka's serve again to take the set.
Rain hit play three games into the second set, but not before Konta had broken her opponent again.
The Briton fought back from a break point in the first game but won all seven on her first serves.
Play is expected to resume at 11am (4pm BST) on Thursday.