(Reuters) - Denmark's Caroline Wozniacki advanced to the last eight of the Family Circle Cup without hitting a shot on Thursday, while matches involving Serena and Venus Williams were postponed after a lengthy rain delay in Charleston, South Carolina.
A rain delay of nearly seven-and-a-half hours forced organizers to push back top seed Serena's round of 16 match with qualifier Mallory Burdette and Venus' battle with Varvara Lepchenko to Friday.
Second seed Wozniacki was given a free pass to the quarterfinals when Andrea Petkovic withdrew with a calf injury just a few hours before their scheduled match.
Third seeded Australian Samantha Stosur went down to Canadian qualifier Eugenie Bouchard, retiring hurt with a leg complaint after losing the first set 6-1 and trailing 2-0 in the second.
Serbia's Jelena Jankovic ousted 19-year-old American Jessica Pegula in just 64 minutes 6-0 6-4 but was affected by the lengthy delays.
The former world number one was too strong for professional debutant Pegula, who won her first two matches before getting completely outclassed in straight sets.
"I wanted to take a nap, but I couldn't," Jankovic told reporters.
"We joked that if they rained it out, we have a shopping day.
"At the end of the day, I was lucky. I came out and played quite well."
Switzerland's Stefanie Voegele prevailed 3-6 6-1 6-3 over 10th seed German Julia Goerges.
Reporting by Ben Everill in Los Angeles. Editing by Patrick Johnston