World number one Serena Williams shook off a woeful start to advance her quest for tennis history while 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 on Wednesday.
"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 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, while top-ranked Novak Djokovic, on a last-eight collision course with Nadal, played later against Austrian Andreas Haider-Maurer.
Three-time defending champion Williams, the oldest women's Slam winner, completed her second "Serena Slam" of four major wins in a row by winning the Wimbledon crown in July.
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 -
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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.
