Serena Williams on returning to the court after childbirth: 'I win, or I don’t play'

— -- With Serena Williams nearing the end of her first pregnancy, the 35-year-old tennis icon opened up about her plans to return to the court early next year -- and under what conditions she would choose to walk away from the game she loves for good.

The 23-time Grand Slam champion announced in April that she is expecting her first baby with her fiance, Alexis Ohanian.

In a new interview with Vogue, Williams admits that she used to think that having children would mean quitting tennis.

"It’s hard to figure out what the end of your tennis career should look like," she told Vogue. "But no. I’m definitely coming back. Walking out there and hearing the crowd, it may seem like nothing. But there’s no better feeling in the world."

In fact, Williams won her last major competition, the Australian Open, while she was a few weeks along in her pregnancy.

Now, she said she plans on returning to defend her Australian Open title in January, even though that will be just three months after she gives birth.

She said she knows returning to the court so soon after childbirth is going to be a big challenge, and adds that if she finds she isn't the player she was at the height of her career, she will hang up her tennis shoes.

"In this game, you can go dark fast. If I lose, and I lose again, it’s like, 'She’s done.' Especially since I’m not 20 years old," Williams told Vogue. "I won’t win less. Either I win, or I don’t play.”

Williams said aside from her career, she is also focused on the health and well-being of her child, and how that factors into life with her fiance and their growing family.

"I’m nervous about childbirth. I’m not a spring chicken," she said. "The biggest thing is that I don’t really think I’m a baby person. Not yet. That’s something I have to work on. I’m so used to me-me-me, taking care of my health, my body, my career. I always ask, 'Am I going to be good enough?'"

Although she knows she'll have to worry about a daughter or son very soon, she told ABC News last month that she was enjoying her pregnancy and taking some time off from tennis.

"I’ve never had this time off before," she said. "I’ve been really enjoying my time off and getting ready for this new, massive chapter in my life."

ABC News' Katie Kindelan contributed to this report.