Serena Williams during her US Open match against Kiki Bertens at USTA Billie Jean King National Tennis Center in New York on September 2, 2015Serena Williams during her US Open match against Kiki Bertens at USTA Billie Jean King National Tennis Center in New York on September 2, 2015 (AFP Photo/Jewel Samad)
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New York (AFP) - Top-ranked Serena Williams spent nearly an hour on the practice courts after struggling through the second round of the US Open, working on her skills after 10 double faults and 34 unforced errors.
Williams, trying to complete the first calendar Grand Slam since Steffi Graf in 1988 and match Graf's Open Era record of 22 career Slam singles crowns, was woeful at times in a 7-6 (7/5), 6-3 victory over 110th-ranked Dutch qualifier Kiki Bertens.
"I definitely feel more determined to do better than what I did," Williams said. "I know I can play better. It definitely wasn't my happiest of moments.
"I don't think you should be happy with just winning. At least I'm not. Maybe other people can. Always looking to do better. If I don't play well, I'm not going to be happy even if I won."
No-one won more this year than Williams, whose 50-2 record tops all women.
She has won the past four Grand Slam titles, completing a "Serena Slam" by winning Wimbledon in July, and at 33 is the oldest Grand Slam women's singles champion.
But in the first set on Wednesday, Williams had to break Bertens in the 10th game, hold in the 11th after committing four double faults and then recover after falling behind 4-0 in the tie-breaker.
"Just trying to get those second serves in, I don't know what was on my mind at that point," Williams said. "I was just trying to keep fighting and keep trying."
She won seven of the last eight tie-break points to swipe the first set after eight double faults and 26 unforced errors.
She broke three times to win the second set and take the match after 92 minutes, but it was a narrower escape than it should have been.
"I just got a little nervous," she said. "But I've been doing totally fine. I've been completely relaxed, chill. I've been really, really fine. So I'm going to get back into the place that I was and I'll be fine again."
The first steps in that direction meant going onto the practice court with coach Patrick Mouratoglou just after her victory.
"Patrick told me some things that he saw that he thought I could work on to improve it and to get better," she said. "I've done it a few times. It's nothing new actually."
Comebacks are also nothing new for Williams, but she knows she cannot put her back against the wall too often against top-level foes.
"When I get down, I tend to get really relaxed and I start to play a little better. Taking it one moment at a time for me," she said. "I've always made some legendary comebacks."
Williams took no solace from having played only 30 minutes in her first match due to a retirement.
"That is no excuse. I just felt I should have been, and am, fine," she said.
"I felt a little tight today. I think I'll do things different, just things with myself and I'll be better for my next match."
Next in her path is fellow American Bethanie Mattek-Sands on Friday.
"She's on a mission to get a record," said Mattek-Sands. "It's going to be a battle out there."
Mattek-Sands, who underwent hip surgery last year, shared Australian and French Open doubles titles with Czech Lucie Safarova.
"Knowing she's capable of having big wins kind of relaxes me because I know she's going to come out and she's going to give 300 percent. She's a huge fighter," Williams said.
"She has a great game. I know that will help me, that I have to start out strong if I want to stay in the tournament. If not, I can go on vacation."