A sweltering day at Montreal took its toll on the first round of the National Bank Open, as three players were forced to retire from their opening-round matches Monday.

Eleventh-seeded Maria Sakkari of Greece advanced when the Czech Republic's Marie Bouzkova was forced to stop while trailing 6-4, 3-1.

Another Czech player, qualifier Tereza Martincova, retired while behind 6-1, 4-3 to U.S. qualifier Amanda Anisimova. Australia's Ajla Tomljanovic won the first set 6-2 against France's Fiona Ferro, then dropped the second set 6-2 before having to stop.

An official heat warning was issued for Montreal during the day as temperatures rose to 102 degrees Fahrenheit (39 degrees Celsius).

Sakkari said of Bouzkova, "It was unfortunate she had to retire," before adding about her own performance, "Being a break down in the first set, then finding a way to win was the key today. I'm very glad I gave myself a chance to play a better match in the second round."

Sakkari, a semifinalist at the French Open, is looking to bounce back from mediocre results at Wimbledon (second-round loss) and the Tokyo Olympics (third-round loss).

With the tournament's top eight seeds, including No. 1 Aryna Sabalenka of Belarus and No. 2 Bianca Andreescu of Canada, receiving first-round byes, only four seeded players were in action Monday.

No. 13 Ons Jabeur of Tunisia routed France's Clara Burel 6-1, 6-3, while France's Oceane Dodin upset No. 14 Karolina Muchova of the Czech Republic 6-3, 1-6, 6-2.

No. 16 Madison Keys of the United States played a late match against Canada's Rebecca Marino.

Jabeur needed just 57 minutes to prevail despite hitting no aces and putting just 56 percent of her first serves in play. She saved three of the four break points she faced and converted five of her eight break opportunities.

Dodin overcame 10 double faults (to just two for Muchova) as she won 71 percent of her first-serve points to Muchova's 58 percent on her first serve.

Also advancing to the second round were Katerina Siniakova of the Czech Republic, Paula Badosa of Spain, Nadia Podoroska of Argentina, Sorana Cirstea of Romania and Anastasia Potapova of Russia.