Four years ago, Serena Williams made it to the final of a U.S. Open. It hasn't happened since, and so much has. She won five other Grand Slam titles. She married. She became a mother. She endured surgery after surgery.

On Saturday, she will return to the final of a U.S. Open.

This one is different, she indicated, and not because winning this one would draw her even with Margaret Court's record of 24 major singles titles. Not because she'll pass Chris Evert for most U.S. Open titles won by a woman, six, if she wins.

It's because of Alexis Olympia Ohanian, the daughter delivered by Caesarean section during last year's Open, before the operations needed to address the postnatal complications. Williams returned to the tour in March, reached the Wimbledon final in July, and now this.

"I was literally fighting for my life in the hospital," she said. "Now only a year later, I'm not training but I'm actually in these finals, in two in a row.

"Honestly, it is remarkable. I couldn't have predicted this at all."

A few weeks short of her 37th birthday, Williams is the third-oldest Grand Slam finalist in the 50 years of the open era. If she wins, she will be the oldest Grand Slam singles champion in that time period. "I'm not a spring chicken," Williams said, "but I still have a very, very bright future."

Williams, a longtime role model for women and minorities, has taken up the cause of working mothers since she returned to the WTA tour in March. Her last match before going on maternity leave was the 2017 Australian Open final, which she won.

Williams' results since she came back have been uneven, with quick exits at a few tournaments. But she has increasingly found her footing and has delivered a dominating performance at the Open, marching into the final while dropping only one set and serving notice that she is back in top-flight form.

Saturday's opponent is Naomi Osaka, a 20-year-old born in Japan, raised in New York and based in Florida who is seeded 20th. Osaka beat Williams in their only meeting, in March. "I was breast-feeding at the time, so it was a totally different situation," Williams said. "Hopefully I won't play like that again. I can only go up from that match."

In her semifinal victory, Osaka repelled 13 break points. She knows where that determination came from.

"I was just thinking I really want to play Serena," she said.

Why?

"She's Serena," Osaka said, as if that explained everything, and maybe it did.

Williams still calls this the beginning of her return from her maternity break. She estimated she's at 50 or 60 percent of peak form and is still getting used to her post-childbirth body.

She weighs less than she did before she became pregnant, "but it's distributed differently now," she said. "I'm still waiting to get to be the Serena that I was, and I don't know if I'll ever be that physically, emotionally, mentally. But I'm on my way."