She and sister Venus boasted that they could beat any male player ranked 200 or lower. A fellow ranked 297 in the world challenged them both and beat them both handily. They later said they could beat anyone ranked 300 or lower, but never again made the mistake of playing a decent male player.
Karsten Braasch didn't just beat them, he beat them back to back 6-1 and 6-2 despite not even warming up. Afterwards he called it that they wouldn't be able to beat someone ranked 500 or better.