Serena Williams reflects on winning the Australian Open while pregnant

Serena Williams of the US hits a return against Venus Williams of the US during the women's singles final on day 13 of the Australian Open tennis tournament in Melbourne on January 28, 2017.

BERLIN — It was an impressive enough sporting feat when Serena Williams won a record 23rd Grand Slam singles title at the 2017 Australian Open, beating her sister Venus in the final. Soon after came a revelation: she had been pregnant at the time.

"I don't know how I did that, honestly," Williams said at an event Wednesday, selecting the moment as the most important of her tennis career.

It's been quite the career — after winning her first Grand Slam at the U.S. Open in 1999, Williams went on to become the only player to have achieved the "Golden Slam" — winning the Australian Open, French Open, Wimbledon and US Open and getting an Olympic gold medal — in both singles and doubles tennis.

"I don't know what I was doing then, I was nine weeks pregnant ... I do remember not being able to run for a long time," Williams said at the SuperReturn private equity conference in Berlin.

"I didn't tell anyone. I mean, Venus knew, and I still feel really bad about that, because a deep part of me feels like because we played each other in the final, I'm like, she must have known, and she must have felt some sort of deep heaviness to go even further and go all out. But she was only one of two people that knew."

Early pregnancy symptoms can be intense as the body undergoes a huge transformation, with a surge in hormones often leading to fatigue, breathlessness and nausea.

"I remember saying, I have to do a lot of aces, I have to do a lot of winners," Williams continued, referring to a point scored directly from a serve. "I can only play, like, four balls and I'm done."

"I remember one time playing a long point against one player, and I couldn't breathe. And I'm like, How does she not see that I'm not able to breathe right now, and so I just intentionally lost the next point just to kind of try and get my energy back. But then I was like: Why am I playing this far pregnant? This is nuts."

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