Re: I'm surprised to see Mariana Alves officiate Serena's GS match
Mariana Alves has been at the US Open nearly every year since 2004. She's a gold badge chair umpire (the very highest reserved for a small few), nonetheless, and is essentially mandated at each grand slam.
I bet she requested to not do Serena's matches (it's the chair umpire who says which players she'd rather not umpire not the player's... though it's often mutual)but now must have been a change of heart.
Umpires make errors. Players make errors. If umpires made only a quarter as many errors as players do in a match, players still would be upset.