World number one Serena Williams shook off a woeful start Wednesday to advance her quest for tennis history while Novak Djokovic and Rafael Nadal joined her in the third round of the US Open.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."The 33-year-old American, trying to match Graf`s Open Era record of 22 career Slam singles titles and win an Open Era-record seventh US Open crown, improved to 50-2 on the year with her 30th Slam match win in a row and sustained her march toward history.Spanish eighth seed Nadal, a 14-time Grand Slam champion, ousted Argentina`s Diego Schwartzman 7-6 (7/5), 6-3, 7-5, for his 750th career tour-level match triumph while top-ranked Djokovic stayed on a last-eight collision course with Nadal by dispatching Austrian Andreas Haider-Maurer 6-4, 6-1, 6-2.The reigning Australian Open and Wimbledon champion, whose only US Open title came in 2011, won 14 of the final 17 games. "From that 4-all moment, I played really well," Djokovic said.Three-time defending champion Williams completed her second "Serena Slam" of four major wins in a row by winning the Wimbledon crown in July, becoming the oldest Slam winner.Williams broke back to level at 5-5 in the first set but double faulted four times in the 11th game before holding, then fell behind 4-0 in the tie-break before rallying largely on Bertens` unforced errors."It definitely doesn`t worry me, being down a lot," Williams said. "I know I can make a comeback, make a run for it."But her form was so shocking that Williams ran to the practice courts to work on serves after the match.Next up for Williams will be fellow American Bethanie Mattek-Sands, who beat compatriot CoCo Vandeweghe 6-2, 6-1."I`ll have to play a little better if I want to win," Williams said. - Nadal `lucky` to advance -Nadal was down 5-4 in the first set tie-breaker before battling back against Schwartzman."Happy to be through," Nadal said. "Was a tough battle. I feel lucky to be through.... To take that first set was very important to me."Nadal, the 2010 and 2013 US Open champion, improved to 22-1 in his past 23 US Open matches after missing 2012 and 2014 due to injury and finishing as the 2011 runner-up.