SERENA Williams misses her daughter Olympia badly. AFP

NEW YORK (AFP) — While Serena Williams chases tennis history at the US Open, she’s also planning a fashion show and spending as much time as she can with baby daughter Olympia.

The 37-year-old American advanced to the fourth round at Flushing Meadows on Friday by defeating Czech Karolina Muchova 6-3, 6-2 in only 74 minutes.

Williams, seeking her 24th Grand Slam title to match the all-time record set by Margaret Court, will play Croatian 22nd seed Petra Martic on Sunday for a quarterfinals berth.

But she’s spending precious rest time preparing for her clothing line’s presentation at Fashion Week, next week’s apparel spectacular in New York.

And the six-time US Open champion finds that the feeling of missing her daughter is tougher than what opponents put her through.

“The toughest thing right now is being away from my daughter. That’s the only thing that’s tough,” said Williams, who often trains early to spend the rest of her day with Olympia.

“In tournament schedule, it’s totally different, so I’m kind of pushed out of my usual day-to-day life — ‘Oh, my God, I’m not with her,’” Williams said.

“That has been the most, I mean ‘the’ toughest, thing I’ve ever dealt with in my career.”

Two years ago, Williams suffered a lung blood clot during labor while giving birth to Olympia, sparking major concerns and a significant hospital stay.

“I don’t really think about it, but it’s cool,” she said. “I think two years ago I had things attached to my arm, IVs, and it was a miracle that happened, having my baby. It was really a great day for me.”

“Then it all went downhill. It all went downhill after that for a good five days. But I’m here and I’m going to be reflecting on my opponent more than anything.”

Now Williams feels sadness at leaving her baby for the tennis court.

“In the beginning she would really be upset when I left. And now she’s a little bit better. I think I’m a little more upset,” Williams said.

“It’s hard. Sometimes my heart literally aches when I’m not around her. But it’s good for me, I guess, to keep working — and just to all moms out there that it’s not easy. It’s really kind of painful sometimes. Sometimes you just have to do what you have to do.”