Olympic:Serena will face Azarenka in semi-finals
The American, who has two doubles gold medals, won 6-0 6-3 as Azarenka of Belarus beat Angelique Kerber 6-4 7-5.
Russia’s Maria Sharapova overcame Kim Clijsters and plays compatriot Maria Kirilenko, who upset Petra Kvitova.
In the men’s last four, Roger Federer meets Juan Martin del Potro and Novak Djokovic takes on Andy Murray.
Swiss top seed Federer posted a 6-4 7-6 (7-5) victory over American John Isner to give himself an Olympic medal shot for the first time.