Upsets bookended the Rogers Cup on Monday with Ukrainian teen Dayana Yastremska beating British 13th seed Johanna Konta 6-3 6-2 early on in Toronto before Maria Sharapova fell 4-6 6-3 6-4 to Anett Kontaveit under the lights in the evening session.
Estonian Kontaveit battled back from a set and a break down in the first-round match to defeat the five-time Grand Slam champion, who entered the tournament as a wild card after injuries kept her out of action for much of the season.
The Russian sent a forehand service return long on match point to end the engrossing two hours and 41 minutes battle.
Kontaveit will next face either seven-time Grand Slam champion Venus Williams or Spain`s Carla Suarez Navarro, who play their first-round match on Tuesday.
Earlier in the day, Yastremska, ranked 33rd in the world, squandered four match points but took her fifth opportunity, dispatching Konta with the clock barely ticking past noon on the Toronto hardcourt. The result avenged Yastremska`s loss to Konta when the pair met at Eastbourne in June.
The 19-year-old from Odessa is having a breakout season after claiming two titles and producing a spirited run to the last 16 at Wimbledon.
Russia`s Daria Kasatkina recovered from a disastrous first set to upset 12th seed Angelique Kerber 0-6 6-2 6-4 while Latvia`s Jelena Ostapenko, 15th seed Caroline Wozniacki and American Jennifer Brady all advanced in straight sets at the WTA`s Premier 5 event.
In other matches, 76th-ranked American Jennifer Brady defeated 50th-ranked Kristina Mladenovic of France 6-1 6-2.
Swiss 11th seed Belinda Benic, the 2015 champion, beat Russian Anastasia Potapova 6-2 6-1 and 20th-ranked Belgian Elise Mertens outlasted Aliaksandra Sasnovich of Belarus 3-6 6-3 6-1.
Tuesday`s schedule features world number one Ashleigh Barty taking on American Sofia Kenin and an all-Canadian clash between Eugenie Bouchard and teenager Bianca Andreescu.