London - World No 1 Victoria Azarenka moved into the Olympic semi-finals with a hard-fought 6-4, 7-5 victory against German seventh seed Angelique Kerber on Thursday.
Belarus's Azarenka took one hour and 46 minutes to subdue the gritty Kerber under the Centre Court roof and will play reigning Wimbledon champion Serena Williams or former world number one Caroline Wozniacki for a place in the final.
Azarenka, who won this year's Australian Open, broke in the second game. But Kerber, who reached the Wimbledon semi-finals this year and shocked Venus Williams in the previous round of the Games, drew level with a break at 3-4.
However, it was Azarenka, a Wimbledon semi-finalist for the last two years, who broke again to take the set.
Neither player seemed able to take control in a chaotic second set which featured seven breaks of serve.
Azarenka had three match points at 5-4, but Kerber saved them all and then broke back.
Remarkably, Azarenka was able to keep her composure despite so many mistakes and she finally sealed the win with one last break at 6-5.