Former world No 1s Karolina Pliskova and Victoria Azarenka advanced after rain delays on Monday's first day of the WTA Montreal Open hardcourt tournament.
Czech standout Pliskova beat China's Zhu Lin to book a second-round berth against current world No 1 Iga Swiatek of Poland, the reigning US and French Open champion.
Pliskova, the runner-up at Montreal in 2021, needed two hours and 22 minutes to eliminate Zhu 6-3, 6-7 (8/10), 6-2.
Rain hit with the tie-breaker level 8-8 after Pliskova had missed on two match-point chances. Zhu won the first two points once play resumed but Pliskova broke her three times in the final set for the victory.
"I was kind of ready for both situations because it's 8-8, so it can just be like two quick points, so you have to be ready that you are going to lose them as well," Pliskova said.
"It's tough because you're so close to the win, and then you just start from zero going in the third set, but there's nothing you can do. You have to still be there and fight."
Swiatek is 2-0 against Pliskova, blanking her 6-0, 6-0 in the 2021 Rome final and prevailing in three sets four months ago in Stuttgart.
"I was not really close to winning (in April) but I just felt like I found the game which I have to play against (Swiatek)," Pliskova said.
"She doesn't give you many free points. You have to find that balance between playing risky, aggressive, but not too risky, not too much."
World number 19 Azarenka, the 16th seed from Belarus, won 6-3, 6-0 over Poland's 25th-ranked Magda Linette in a matchup of this year's Australian Open semifinal losers.
It was Azarenka's 187th career match victory at the WTA 1000 level, the 34-year-old breaking the former mark set by Simona Halep for the most wins since the level began in 2009.
Azarenka, a two-time Australian Open champion, returned from a three-hour rain delay between sets and won 24 of 31 points in the second set, avenging a loss at Miami to Linette.
US 13th seed Madison Keys, the 2017 US Open runner-up, defeated 43-year-old compatriot Venus Williams, a seven-time Grand Slam champion, by 6-2, 7-5 after an hour and 51 minutes.
Keys, 28, won her seventh career WTA title last month at Eastbourne. She booked a second-round match against Italy's 49th-ranked Jasmine Paolini, who eliminated world number 22 Donna Vekic of Croatia 7-6 (7/3), 6-2.