Petra Kvitova will join Garbiñe Muguruza on the sidelines of Rogers Cup.
The Czech says her left arm injury is still not healed enough to allow her to begin the hardcourt season.
Kvitova missed Roland Garros and all the of the grass-court tournaments before Wimbledon because of the injury. She elected to play Wimbledon and reached the fourth round, but the injury was never really perfectly healed.
Kvitova joins Muguruza, who also had to pull out of San Jose with an arm injury.
The withdrawals mean that Serena Williams will get bumped up to No.8 seed and Venus Williams will be entered in the main draw.