"Evert and Navratilova, you say, both would have won many more titles, were it not for the existence of the other. OK, but how many more Slams would Serena have won if she didn't have to face her sister, infinitely weirder than a non-blood-related rival? Serena is playing during a "soft" period in tennis history, you say, as evidenced by her winning Wimbledon without being tested. I disagree. Serena has often had to go through the best to win (Davenport, Sharapova, Venus, Henin). Is, say, Vera Zvonareva, really a worse player than the Helena Sukovas or Zina Garrisons that other contenders often faced in finals?
Serena has the most fearsome serve -- i.e. the most important stroke -- in women's tennis history, and it would be the case even if everyone used the same technology. (She uses natural gut strings by the way.) She is the best athlete in women's tennis history, likely the fastest and the strongest. And she competes as well as any athlete -- not tennis player; athlete -- you'll ever come across.
But here's where I really feel strongly: Head-to-head, on a neutral surface (i.e. hard courts), everyone at their best, I can't help feeling that she crushes the other legends. Sacrilege, I know. But spark up of video of other players, watch where their balls land in the court or how hard they serve or how they move and then consider Serena's game. She would blow through Evert. She wouldn't allow Navratilova (who looks like a pixie next to Serena) a chance to attack. She would tee off on Graf's slice. Again, this isn't to disrespect the others; it's progress. But I think it counts for a lot that no one has ever played tennis at a higher level than Serena has. (It's the same reason, incidentally, that I was early to pronounce Federer the male GOAT. You just know watching him that no one has played better tennis qualitatively and surely that has to count for something.)"
Jon Wertheim from sports illustrated