Serena Williams battled through her own mistakes and the tension of her quest for tennis history on Friday to reach the US Open fourth round.

The 33-year-old American fought back to defeat 101st-ranked compatriot Bethanie Mattek-Sands 3-6 7-5 6-0 at Arthur Ashe Stadium and needs only four more triumphs to be the first player since Steffi Graf in 1988 to win the Grand Slam.

Williams struggled with her serve early and made half her 28 unforced errors in the first set, but found her form late in the second set and dominated from there.

“I’m not trying to live on the edge,” Williams said.

“I don’t think I came out too slow. I think Bethanie came out really well. I had to adapt to her game and I finally got some rhythm going toward the end of the second set.”

Williams, holder of all four major titles, also seeks her 22nd career Slam singles crown to match Graf’s Open Era record and move two shy of Australian Margaret Court’s all-time mark.

Williams, who could reach the final without facing a top-10 foe, next plays US 19th seed Madison Keys, who beat 15th seed Agnieszka Radwanska 6-3 6-2.

“I only have one match I’m looking forward to and that’s the next one,” Williams said.

“This journey has been great. Every night I come out it feels better and better.”

Mattek-Sands, her blonde hair tinted with tangerine dye, took charge early, leading 3-0 and breaking back to 4-2, then holding twice to claim a first set where Williams was 1-7 on break chances.

It was not unfamiliar territory as Williams went to three sets for the 10th time in 24 slam matches this year and stayed perfect.

Older sister Venus Williams, 35, eased Serena’s path to the final by ousting the top-rated rival on that side of the draw, defeating 12th-seeded Swiss teen Belinda Bencic 6-3 6-4.

Bencic, 18, inflicted the most recent defeat upon Serena last month in a Toronto semi-final, but fell to 0-4 against Venus.

Venus, 35, fired 31 winners against 15 unforced errors while Bencic had 12 of each.

“I just wanted to be aggressive,” Venus said. “Keeping the errors down really helped me close out the match.”

Venus, seeded 23rd, reached a fourth-round match against 152nd-ranked Estonian 19-year-old qualifier Anett Kontaveit and could face Serena after that.

“I hope we both get to the quarter-finals,” Venus said.

13th seed Ekaterina Makarova of Russia fought off cramping to oust 17th seed Elina Svitolina 6-3 7-5 and is now the top-ranked rival in the Williams half of the draw.

Makarova, playing with a taped upper right leg, began cramping while serving at match point. Told she could not see a trainer until a changeover, she fought on and advanced on a forehand winner.

“I was scared,” Makarova said.

“It’s the first time I was cramping. She told me she can’t treat me until the changeover. I kept playing and kept trying.”

Also making it to the fourth round is Canadian star Eugenie Bouchard, who put together three straight wins for the first time since making the Australian Open quarter-finals back in January.

Bouchard defeated diminutive Slovakian and 2014 Australian Open finalist Dominika Cibulkova 7-6 (11-9) 4-6 6-3 and will face Italy’s Roberta Vinci in the round of 16.

Also through to the fourth round is rising French star Kristina Mladenovic. The 22-year-old breezed past Russian Daria Kasatkina 6-2 6-3.