R. Schweikart
Professional
Serena is a level higher in power then hingis/navratilova.
Graf in old age would have had no chance, it would have been a bloodbath.
Considering that Serena in the 00s lost to Likhovtseva, Craybas, Hantuchova, Srebotnik, Jankovic, Kuznetsova and four times to Capriati (!) in slams I think it would have been the Compton girl's blood which would have been spilled.
Steffi was far to quick for Serena and her slice would have driven Serena nuts.
OK, Serena might have won 30 % of their matches (comparable to Seles, Sabatini types) but that's it. She was far to inconsistent in the 00s. Found her groove only in the 2010s when all big opponents were gone.