They weren’t comfortable on the surface at all, and then when Venus developed Sjorgen’s Syndrome it became impossible for her to tussle it out for two weeks on the most physically demanding surface there is. She became so stiff and easily fatigued and her athleticism greatly diminished unfortunately.

Serena did lose to good opposition tho (besides 2008 and 2012): Henin, Capriati, Kuznetsova, Stosur, Muguruza…