The top-ranked Williams trailed much of the first set but rallied and beat Peng Shuai 7-5, 6-2 Sunday night in the third round at the Sony Ericsson Open.
Williams shares the tournament record of five titles with Steffi Graf and has won the title the past two years. She has lost only one match at Key Biscayne since 2001.
Against Peng, Williams fell behind 5-4, then won seven of the next eight games. She struggled with her second serve but smacked seven aces.
Williams will play for the third day in a row Monday, when her opponent in the round of 16 will be No. 17-seeded Zheng Jie.
