Serena Williams’ U.S. Open win over Kaia Kanepi was tough, probably tougher than expected. Just like her climb back from childbirth to dominance.

But Williams didn’t back down from either challenge. Still seeking her first Grand Slam title since motherhood, she fought her way to a taxing 6-0, 4-6, 6-3 fourth-round win over Kanepi and into Tuesday’s quarterfinals.

“It wasn’t an easy match at all. She obviously knows how to play. She’s had a lot of big wins in her career. I was just happy to get through it, to be honest,” Williams said in a TV interview on court at Arthur Ashe Stadium. “I’ve literally grown up on this court. That’s super special. I played here at 15 or 16, and I’m still going.”

Williams is still going, on to a matchup with eighth-seeded Karolina Pliskova. She had to survive upset-minded Kanepi, who knocked off No. 1 Simona Halep in the first round and stormed back from being bageled in the first set to seize the second. The 33-year-old Estonian pushed Williams, but the six-time champ relied on her serve — and her nerve — to hold on.

After Kanepi broke early to go up 2-0 in the second, a rare double-fault by Williams handed the Estonian a break and a 5-2 lead. She held on to win the second set.

But in the final set, Williams broke early to go up 2-0. She fended Kanepi off a break point, holding serve and roaring. She punctuated her 17th ace of the game with a huge “Come on!” to go up 5-2. And after getting to triple match point, she brought Kanepi to net and won with a passing shot.

“Today [her serve] was unreturnable,” Kanepi said.

It was close to that. Williams hit 18 aces and 47 winners and showed even better conditioning and court movement than she displayed in her Wimbledon finals run.

“The thing I’ve improved most is my fitness,” Williams said. “Coming back from a baby is really hard, harder than I thought. That was the most important thing for me to improve, being fit and able to play long matches.”

She may need to in order to win this Open, which would make history. A seventh win here would break her tie with Chris Evert for the Open Era record, and her 24th slam title would pull her even with Margaret Court’s all-time record. Doing it after motherhood would be all the more impressive.

“It’d be wonderful to win a Grand Slam while having a daughter, but it’s a lot of work. It takes a lot of work to win those tournaments. I’m only in my third one back,” said Williams, who won the 2017 Australian Open while pregnant with her daughter, Olympia. She hasn’t won since and is trying to avoid going without a Grand Slam for the first year since 2011.

“Society puts it out there that you’ll just kind of snap back, and that’s just a myth,” Williams said. “I feel like it’s important for women to know that it doesn’t happen like [it looks] in the Instagram world. But in the real world, it takes a while for your body to come back. Especially after a C-section, it takes a little bit longer.

“Not only that, mentally and physically and dealing emotionally with providing for another child, a lot that goes into it. I was just living in this world where I thought it’d just automatically come together — yeah, my dress, I kind of look like a magical fairy — but it’s not happening, so I had to realize and live in reality.”