I think Maria just isn't tough enough for Serena, who's a true great and tough as teak. Serena's durability is remarkable, but kind of like Agassi, she has some in the tank because she didn't use it earlier. But her recoverary since 2011 is special and very welcome. Women's tennis is dire and Serena brings quality to it, which is sadly lacks.
I hope she's around for a couple more years and continues to win majors. She's the greatest of this era, without a doubt. One of the greats...