I have NO problem with the foot fault call, though the foot fault wasn't egregious. What I do have a problem with is the lineswoman tattling on Serena. When it's match point at the US Open, you need to make sure that the match ends the way it's supposed to end, not on a Code Violation. It's really bad for the game and really bad for the tournament, and really unfortunate for the fans because it was a close match.
Obviously this is a very subjective issue because you can easily argue that "technically" Serena deserved a Code Violation after the first set. But I find it really hard for someone to make the case that Serena deserved a Code Violation for losing her cool towards a lineswoman, especially in THAT situation. The lineswoman had every right to call a foot fault. But I've seen many a match where a linesperson will make a call and just sit there and take it when a player reads them the riot act.
It's chicken**it to end an important match that way, with the lineswoman scampering over to the chair umpire to tattle, and then after that, when Serena gets upset at her again, she goes BACK to the chair umpire like Serena was going to do something to her. Comeon....was she really going to hurt her in front of all those people. How can she feel threatened. The faint of heart should not be linespeople. Also......boo on the chair umpire for bringing the tournament referee into the picture. Chair umpires are supposed to use discretion, not simply be drones.
Serena should have been able to finish the match and then substantially fined by the tour afterwards.