New York: World number one Serena Williams shook off a woeful start Wednesday to advance her quest for tennis history while Rafael Nadal joined her in the third round of the US Open.

Serena Williams. Pic/AFP
Serena Williams. Pic/AFP

Williams, trying to complete the first calendar Grand Slam singles sweep since Steffi Graf in 1988, defeated 110th-ranked Dutch qualifier Kiki Bertens 7-6 (7/5), 6-3 at Arthur Ashe Stadium despite 34 unforced errors and 10 double faults.

"I just kept fighting for each point, not for a lot but just one at a time," Williams said.

"I had been pretty relaxed. Today I was a little tight. I think it showed. Hopefully, I can get back to where I was before."

Williams is trying to complete the first calendar Grand Slam since Steffi Graf 27 years ago and equal Graf's Open Era record of 22 Slam singles titles, two shy of Australian Margaret Court's all-time record.

Williams completed her second "Serena Slam" of four major wins in a row by winning the Wimbledon.