Williams and Isner combined to Anderson and Scheepers 6-3 6-2 to secure victory in the mixed team event.
CNN: STORY HIGHLIGHTS
2.
Williams, who has battled injuries and illness, came from a set down to beat Chanelle Scheepers 4-6 6-2 6-3 in the opening rubber.
CNN: STORY HIGHLIGHTS
3.
Petrova will play Elena Dementieva in the last eight after her compatriot delivered a sure-footed performance to see off South African qualifier Chanelle Scheepers 6-1 6-3.