NEW YORK, United States (AFP) — Serena Williams struggled but moved nearer to the first calendar Grand Slam since 1988 by defeating Dutch qualifier Kiki Bertens 7-6 (7/5), 6-3 yesterday at the US Open.

World number one Williams, a three-time defending champion seeking her seventh US Open title overall, was outplayed early by her 110th-ranked rival at Arthur Ashe Stadium.

But Williams overcame 34 unforced errors and 10 double faults to win and sustain her march toward history.

“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 is trying to complete the first calendar Grand Slam since Steffi Graf in 1988 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 crown in July, and the American can claim her Open Era-record seventh US Open title as well as extend her mark as the oldest woman Grand Slam champion.

Bertens nearly derailed all those dreams with more than a little help from Williams.

After breaking Williams with a running forehand winner for a 2-1 lead, Bertens attacked and made tough shots while Williams sprayed unforced errors and was inconsistent with her serves.

Bertens served for the first set at 5-4, but struggled with the pressure of the moment and Williams broke back to level, only to double fault four times in the 11th game, which lasted more than eight minutes, before holding.

“I can’t find it,” Williams yelled toward Coach Patrick Mouratoglou sitting in the stands, as she searched for top-serving form ahead of the tiebreaker.

Bertens rolled to a 4-0 lead in the tiebreaker, but again faltered with the set within reach, surrendering the next five points, four on errant forehands, before Williams netted a backhand return to level the decider at 5-5.

Bertens netted a backhand to give Williams a set point and the US star had a net-cord ball drop on the sideline. Bertens swatted a desperate forehand long and Williams had taken the set, reacting by bending forward and screaming with fists clenched and her body shaking.

“It definitely doesn’t worry me being down a lot,” Williams said. “I know I can make a comeback, make a run for it.”

After an early exchange of breaks in the second set, Bertens double faulted away a break to give Williams a 3-2 edge and she broke again in the last game to finish matters after 92 minutes.

Next up for Williams will be 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.

With three top-10 seeds on Serena’s side of the draw losing openers and third seed Maria Sharapova withdrawing due to injury, Williams could reach a Grand Slam singles final without facing a top-10 rival for the first time in her career.

Swiss teen Belinda Bencic, the highest seed remaining in Williams’ half of the draw at 12, was to face Japan’s 88th-ranked Misaki Doi later yesterday.

Bencic, who defeated Williams in the Toronto semi-finals last month, could play Serena’s older sister Venus, who meets fellow American Irina Falconi later, in the third round.

Either Bencic, 18, or Venus, 35, could meet Serena in the quarter-finals.

Australian Open semi-finalist Madison Keys ripped 100th-ranked Czech Tereza Smitkova 6-1, 6-2. The American 19th seed hopes for a fourth-round date with Serena Williams who ousted her in Australia.

“Fingers crossed, it could happen. If it does happen I could come out and have some fun at my home slam,” Keys said.

Top-ranked Novak Djokovic, the reigning Wimbledon and Australian Open champion, was trying to take another step toward his fifth US Open final in six tries later yesterday against Austrian Andreas Haider-Maurer.

Djokovic, whose lone US Open title came in 2011, is on a quarter-final collision course with 14-time Grand Slam champion Rafael Nadal, the Spanish eighth seed who faces Argentina’s Diego Schwartzman.

Defending champion Marin Cilic and seventh seed David Ferrer advanced nearer a fourth-round meeting.

Croatian ninth seed Cilic fired 19 aces and advanced to the third round by defeating 139th-ranked Russian qualifier Evgeny Donskoy 6-2, 6-3, 7-5.

“It’s sort of unreal. I was working all my life to win a Grand Slam and now I’m working to defend one,” Cilic said.

Spain’s Ferrer, the 2013 French Open runner-up who missed the past 2 1/2 months with an elbow injury, downed 102nd-ranked Serb Filip Krajinovic 7-5, 7-5, 7-6 (7/4).

“I’m very happy I’m in the third round,” said Ferrer. “It’s a nice comeback playing on these courts.”