Top-seeded Juan Martin del Potro had an easy straight-sets win in his opening match at the Sydney International on Wednesday, while second-seeded John Isner was beaten by fellow American Bobby Reynolds.
Del Potro defeated Lukasz Kubot 6-4, 6-2 in their second-round match. Reynolds beat Isner 3-6, 6-4, 6-3.
Denis Istomin of Uzbekistsan also advanced with a 6-2, 6-1 win over American Ryan Sweeting.
The women's quarterfinals were scheduled for later Wednesday.
Wimbledon champion Petra Kvitova was to play Daniela Hantuchova, last week's finalist in Brisbane, in the first evening match on center court, followed by top-ranked Caroline Wozniacki against Agnieszka Radwanska.