She notched her 20th major at the French Open and is still dominant at 33 going on 34 years of age. Graf was great but if Seles didn't get stabbed, Graf would've have less majors.
It's difficult to compare men and women, but Serena is dominant and at an advanced age for tennis. There were some baseball players that were dominant at an advanced age and it was PED-related. I don't think that's the case here.
Point being, she'll go down as one of the most underrated athletes of all-time. Any opinions? Maybe the level of opposition isn't the greatest, but she's pushing 34.