I don't recall if it was during an interview or perhaps just in her twitter timeline.
....and after looking for it, the closest thing I can find is her saying she couldn't compete against the top men...how someone like Federer is simply too fast, strong, etc. It's a whole other game.
So while I might be remembering her wrong or thinking of someone else's comments at the time of the spat between Williams and McEnroe....luckily, we don't have to wonder or question if I'm correct.
In 1998 when her sister Venus was top 5 and she was ranked 20 (so admittedly they hadn't reached the peak of their ability) they were in Australia and foolishly said they could beat any man ranked around 200th. That in of itself is a bit of an admission they can't compete with the top men....but still a bit insulting to a guy ranked 200th. The guy at the time ranked 203, who I've never heard of, who never ranked higher than 40th, spoke up and said he'd take that challenge.
The next day, he played a round of golf, and then smoked Venus and Serena Williams back to back....while drinking beer and smoking cigarettes.
I can understand why a woman who never competed against a man at the top of her game may not understand the difference. I can understand why people who never played competitive sports or don't ever follow any sports might imagine there's a possibility of women competing against men at the highest level of some sport....but I can't think of any.
If you look down the records of who broke what record and pooled all male and female competitors together....there's no women at the top of anything that men compete in.
The women's world cup winners....the best female soccer players in the world....lost to a team of teenage boys. I'm talking about 15yo guys....versus the best women in the sport at that time. I think it was a high school boys team. You can watch it yourself, the women aren't going easy....they're trying hard to avoid embarrassing themselves. There's no point in allowing biological men to compete against women.
Now....if that's unfair to trans people who genuinely want to be able to participate in sports, I'd agree, and I think they should be allowed to compete against men. That way we need not worry about ftm testosterone levels or if mtf have all the advantages of male puberty. They can still compete and if they manage to win....nobody will question the fairness or validity of their victory.