CHARLESTON, S.C. -- Matches for top-ranked Serena Williams and her sister, Venus, at the Family Circle Cup were postponed Thursday after a rain delay of nearly 7 1/2 hours.

The long wait wiped out most matches in the busy round of 16, including those of the tournament's headling Williams' sisters.

Serena, riding an 11-match winning streak here, was scheduled to play American qualifier Mallory Burdette. Venus will take on Varvara Lepchenko for a spot in the quarterfinals. Second-seeded Caroline Wozniacki moved on without hitting a ground-stroke when opponent Andrea Petkovic withdrew with a calf injury a few hours before their scheduled match.

The rain fell steadily at the Family Circle Tennis Center until dinner time when crews were finally able to start drying the large puddles on the gloppy, green clay surfaces.

Once play began, ninth-seeded Jelena Jankovic - the 2007 champion here - needed just 64 minutes to oust 19-year-old American Jessica Pegula in straight sets. Pegula, whose father Ted owns the Buffalo Sabres, won the first two professional matches of her career this week before falling to the former world No. 1, 6-0, 6-4.

Third-seeded Samantha Stosur and 10th-seeded Julia Goerges were scheduled to play later Thursday.

The delay means the year's first clay-court event has some ground to make up to finish by Sunday. Without further rain, there would still be four round-of-16 matches to be played on Friday before the quarterfinals were set.

Serena Williams, who was not sharp despite winning her opening match over Camila Giorgi here on Tuesday , will have waited almost three days to play again. Venus defeated 19-year-old Monica Puig in three sets Wednesday night. The sisters would each need to win twice more to set up an all-Williams semifinal here. They haven't met in a tournament since 2009.

Both have enjoyed championship moments at the Family Circle. Serena won titles in 2008 and 2012. Venus won the crown in 2004 and last September clinched the winning points as the Washington Kastles took the World Team Tennis title on Billie Jean King Court.

Others who had matches delayed were sixth-seeded Lucie Safarova against 11th-seeded Sorana Cirstea and the all-American match of Madison Keys and Betthanie Mattek-Sands.

Cirstea, like Serena Williams, also hasn't played since winning on Tuesday.

Jankovic said it was difficult keeping sharp and loose as the delay stretched through the afternoon. "I wanted to take a nap, but I couldn't," she said. "We joked that if they rained it out, we have a go shopping day."

Instead, Jankovic felt fortunate to move to the quarterfinals when so many other competitors will likely have to play twice Friday. "At the end of the day, I was lucky," she said. "I came out and played quite well."

Petkovic was Jankovic's doubles partner this week. Jankovic had enjoyed the pairing so far and looked forward to seeing what they could to here. Instead, Petkovic said in a statement she might do more damage to her leg if she competed.

"It is a very beautiful tournament and I loved every part of it here," Petkovic said. "So I hope to come back under better conditions."

• Read more articles by PETE IACOBELLI

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