Young Canadian Eugenie Bouchard didn t care why she was dominating a Grand Slam champion and top-10 player, only that she had finally broken through for the biggest win of her career.

Bouchard, 19, advanced to the quarterfinals of the Family Circle Cup on Thursday when former U.S. Open champion Samantha Stosur retired during the second set of their match with an apparent injury to her right leg. Stosur was trailing 1-6, 0-2 when she quit.Bouchard, the reigning Wimbledon Junior singles champion, noticed the wrap on Stosur s leg and that her opponent wasn t chasing down balls she would normally reach with ease. Still, Bouchard felt she d been playing a strong match.

"I believe that I can beat these players," Bouchard said. "It s time to happen."An earlier rain delay of nearly 7½ hours washed out matches featuring the tournament s top draws: Serena Williams and her sister Venus.Serena, riding an 11-match winning streak in Charleston, was scheduled to play American qualifier Mallory Burdette, while Venus was to face Varvara Lepchenko for a spot in the quarterfinals. Both matches were rescheduled for Friday.

Former world No. 1 Jelena Jankovic advanced on Thursday, needing just 64 minutes to oust 19-year-old American Jessica Pegula in straight sets. Pegula won the first two professional matches of her career this week before falling to the Serb, 6-0, 6-4.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."

Second-seeded Caroline Wozniacki of Denmark also moved on without hitting a groundstroke when Andrea Petkovic of Germany withdrew with a calf injury a few hours before their scheduled match.In the only other match completed Thursday, Stefanie Voegele of Switzerland defeated 10th-seeded Julia Goerges of Germany, 3-6, 6-1, 6-3.The rain fell steadily at the Family Circle Tennis Center until early evening when crews were finally able to start drying the large puddles on the gloppy, green clay courts.

The long delay means the year s first clay-court event has some ground to make up to finish by Sunday. There are still four third-round matches to be played on Friday before the quarterfinals are set.Serena Williams, who was not sharp despite winning her opening match over Camila Giorgi 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 Cup. 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.Other delayed matches included sixth-seeded Lucie Safarova against 11th-seeded Sorana Cirstea and the all-American encounter between Madison Keys and Bethanie Mattek-Sands.