Andy Murray and Rafael Nadal were winners but six seeds were beaten in a day of upsets in the second round at the Rogers Cup in Montreal.
Number two seed Murray, who received a first-round bye, avoided the fate of so many other seeds, though he was extended by Spain's Tommy Robredo before coming through for a 6-4 7-5 win.
Spain's Nadal, in his first hardcourt appearance since March, showed few signs of rust as he used a dominating first serve to dust off Sergiy Stakhovsky of Ukraine 7-6, 6-3.
Next up for Murray is unseeded Gilles Muller of Luxembourg, who beat 15th seed Gael Monfils of France in a third-set tiebreak 6-3 3-6 7-6(4).
Other seeds to fall were Czech Tomas Berdych (number five), Croatian Marin Cilic (six), Canadian Milos Raonic (eight), Frenchman Gilles Simon (nine) and Bulgarian Grigor Dimitrov (14).
Fourth seed Kei Nishikori also advanced with a 6-3 6-3 win over Spaniard Pablo Andujar.
|Wozniacki blasted WTA rules following her close loss [Nick Turchiaro-USA TODAY Sports]
Meanwhile, world number five Caroline Wozniacki suffered a second round upset to Swiss Belinda Bencic then hit back with strong criticism of the WTA.
Wozniacki played despite recovering from a nagging calf injury and fell in a close match to Bencic 7-5 7-5 before indicating she felt pushed to play by the WTA rules.
"The rules, if you're a top-10 player are that this was my commitment tournament, you're forced to play," Wozniacki told reporters.
"If you don't, you get huge money fines. You get zero points to your ranking ...which is crazy."
Wozniacki, a former world number one, was also defeated in the second round last week in Stanford as she battled injury.
Wozniacki was not alone in being upset in Toronto as third seed Petra Kvitova also went down, losing to former worlkd number one Victoria Azarenka 6-2 6-3.
Defending champion Agnieszka Radwanska avoided such a fate with her 7-5 6-3 triumph against Julia Goerges while second seed Simona Halep shook off the rust and stormed into the third round with a commanding 6-3 6-4 win over former world number one Jelena Jankovic.