Defending champion Caroline Wozniacki is out of the Rogers Cup in Toronto after losing to Roberta Vinci 6-4 7-5 in the second round on Wednesday.
The world number one struggled against Italy's world number 22, sending her return long on match point.
Serena Williams, who spent 123 weeks at the top of the rankings, beat Julia Goerges 6-1 7-6 (7-5).
Wimbledon champion Petra Kvitova also made the third round with a 7-6 6-3 win over Spain's Anabel Medina Garrigues.
French Open winner Li Na advanced when Chinese compatriot Peng Shuai withdrew from their match with a hip injury, while Maria Sharapova edged out 19-year-old Serbian Bojana Jovanovski 6-1 7-5.
Wozniacki, who finished with seven double-faults, admitted she had struggled in blustery conditions.
"It is not fun to lose this, but there's nothing really I can do about it now, just practise, and try to do better," she said.
"It was very difficult at first with the wind. Especially when you threw it up. It was going everywhere, so definitely that didn't make it easy.
"It was blowing in different directions, but it's the same for both players."
Wozniacki's exit leaves the tournament without its top two seeds after with a stomach muscle injury that casts doubts on her upcoming US Open title defence.
Vinci's fellow Italian, eighth-seeded Francesca Schiavone, also went through with a 6-1 6-3 win over Russia's Ekaterina Makarova.