At the Roger's Cup, she lost in the first round to 13th seed Kirsten Flipkens in three sets. At the Western and Southern Open, she defeated qualifier Jana Čepelová in straight sets, before losing in the second round to Elena Vesnina in three sets. Her next tournament was the 2013 U.S. Open. She pulled an upset in the first round by defeating 12th seed Kirsten Flipkens in a rematch of the Roger's Cup first round. She was defeated by Zheng Jie in three sets. She entered the doubles with Serena Williams. Their run ended in the semifinals against the fifth seeded team and eventual champions Lucie Hradecká and Andrea Hlaváčková. Her next tournament was the Pan Pacific Open. In the quarterfinals she defeated Canadian Eugenie Bouchard in three sets, but fell in the semifinals to Petra Kvitová in another three set match. Williams subsequently played at the 2013 China Open in Beijing where she played singles and doubles. Williams lost her second round match in singles losing to Sabine Lisicki and she also lost her first round match in doubles despite having two match points. Williams's last tournament of the season was the 2013 Kremlin Cup in Moscow, but she withdrew due to injury, bringing an end to her 2013 season.
At the Roger's Cup, she lost in the first round to 13th seed Kirsten Flipkens in three sets. At the Western and Southern Open, she defeated qualifier Jana Čepelová in straight sets, before losing in the second round to Elena Vesnina in three sets. Her next tournament was the 2013 U.S. Open. She pulled an upset in the first round by def,eating 12th seed Kirsten Flipkens in a rematch of the Roger's Cup first round. She was defeated by Zheng Jie in three sets. She entered the doubles with Serena Williams. Their run ended in the semifinals against the fifth seeded team and eventual champions Lucie Hradecká and Andrea Hlaváčková. Her next tournament was the Pan Pacific Open. In the quarterfinals she defeated Canadian Eugenie Bouchard in three sets, but fell in the semifinals to Petra Kvitová in another three set match. Williams subsequently played at the 2013 China Open in Beijing where she played singles and doubles. Williams lost her second round match in singles losing to Sabine Lisicki and she also lost her first round match in doubles despite having two match points. Williams's last tournament of the season was the 2013 Kremlin Cup in Moscow, but she withdrew due to injury, bringing an end to her 2013 season.