Petra Kvitova, of the Czech Republic, waves to the crowd following her first round match against Magda Linette, of Poland. -AP

Former Grand Slam champions Petra Kvitova and Samantha Stosur were unfazed by opening-day weather delays, cruising into the second round of the WTA hardcourt tournament in Montreal on Monday.

Nick Kyrgios turned in a patchy performance to exit the Toronto Masters 7-6 (7/2), 3-6, 6-3 at the hands of Wimbledon junior champion Denis Shapovalov.

The 17-year-old Canadian set up a match point with his 12th ace, then calmly closed out the upset over the 19th-ranked Australian.

Two-time Wimbledon champion Kvitova, seeded 12th, needed just 59 minutes to get past Poland’s Magda Linette 6-1, 6-2.

The Czech, who won her first North American hardcourt title in Montreal in 2012, didn’t face a break point against the young Pole, who pushed former world number one Venus Williams to three sets in Stanford last week.

Kvitova, playing her first tournament since Wimbledon, broke Linette twice in each set and finished her off with an ace.

Former US Open champion Stosur, seeded 13th, had to work a little harder to dispatch Britain’s Heather Watson 7-5, 6-3.

After some first-set struggles, however, she broke Watson three times in the second to wrap up the win. A string of rain delays stopped play for a total of four hours.

Organisers eventually pushed the scheduled last match of the night, between Canadian Francoise Abanda and Chinese qualifier Zheng Saisai to Tuesday.

The only other seed in action, 17th-seeded Elina Svitolina, advanced with a 6-2, 6-4 victory over American qualifier Jennifer Brady.

The top eight seeds —now led by second-seeded Angelique Kerber of Germany after the late injury withdrawal of world number one Serena Williams — have first round byes.

Croatia’s Mirjana Lucic-Baroni booked a second-round meeting with Wimbledon finalist Kerber with a 6-2, 6-0 victory over Japanense player Nao Hibino.

Britain’s Naomi Broady beat Puerto Rico’s Monica Puig 3-6, 6-4, 6-2 to line up a clash with third-seeded French Open champion Garbine Muguruza — who was installed at the top of the revised draw after Serena pulled out with inflammation in her shoulder.