NEW YORK: Serena Williams took her second step in a drive to complete the calendar-year Grand Slam but a lackluster performance from the service line sent her straight to the practice court on Wednesday.

Williams, who would be the first woman in 27 years to win all four slam singles titles in the same year, beat Kiki Bertens of the Netherlands 7-6(5) 6-3 despite 10 double faults, including eight in the opening set of their second-round match.

"Just trying to get those second serves in," Williams said about hitting the practice courts with her coach right after the Arthur Ashe Stadium match.

"Patrick (Mouratoglou) told me some things that he saw that he thought I could work on to improve it and to get better."

Williams, 33, was on the ropes twice in the first set and appeared a little deflated when she met with reporters.

"You know, if I don't play well, I'm not going to be happy even if I won," said the American, whose confidence still runs deep with a 50-2 match record this year to back her up.

"I don't think you should be happy with just winning. At least I'm not. I'm always looking to do better."

The world number one has looked vulnerable even as she charged to this year's Australian, French and Wimbledon titles. She lost nine sets along the way, including seven first-set losses.

"I've always made some legendary comebacks since 1998 when it first started," the top seed said. "That's kind of just been my MO (modus operandi) for my whole career."

The valiant, 110th-ranked Dutch player was serving for the opening set at 5-4 when nerves set in for Bertens, and Williams stepped up to break at 15 on the way to forcing a tiebreaker.

Bertens won the first four points in the tiebreak, but once again the American responded. Williams won the next five points and went on to claim it 7-5.

Williams got back on firmer footing in the second set, breaking Bertens three times to book her spot in the third round against compatriot Bethanie Mattek-Sands.

Williams said she relaxes when she finds herself in trouble.

"When I get down, I tend to get really relaxed and I start to play a little better," she said.

Williams said the pressure to join Maureen Connolly (1953), Margaret Court (1970) and Steffi Graf (1988) as the only female Grand Slam winners affected her a bit on Wednesday.

"Until today I was OK with it. I just got a little nervous today," she said. "I've been doing totally fine. I've been completely relaxed, chill.

"So I'm going to get back into the place that I was and I'll be fine again."