Serena is just better. Then again, Serena is better than anyone else, but she can still lose. That being said, when she loses, she doesn't play her best and versus Masha she usually BRINGS it. So obviously, Maria has no chance.
However, I'm upset that she gets a beatdown every time they play. She should go for her shots more, serve well, mix up her game and not be scared. But she isn't changing anything in her game vs Serena therefore the outcome is always the same