Williams has won titles in Charleston, S.C., and Madrid -- plus two victories in Fed Cup.
Considering all the tennis Serena Williams has played during her streak, she didn't have much time after finishing her first-round win over Galina Voskoboeva near midnight Tuesday.
"It wasn't that easy. It was a quick turnaround," Serena said. "When I saw the schedule, I said, 'Oh my God, are you serious?' That's life. You just have to be able to adjust and make the best of it, and so I was really happy to come through today."
However, Serena appeared to struggle with her back at times.
"Everyone has aches and pains here and there, and today I felt it a lot because I have been playing a lot and practicing even more," she said.
Williams next faces fifth-seeded Sam Stosur, who defeated Sara Errani 6-3, 7-5.
Radwanska was beaten by 28th-ranked Petra Cetkovska of the Czech Republic 6-4, 4-6, 6-1; Wozniacki retired with upper respiratory illness while trailing 6-4, 4-0 to Anabel Medina Garrigues of Spain; and Bartoli was eliminated 6-3, 6-1 by Julia Goerges of Germany.
Information from The Associated Press was used in this report.