Watching the highlight it looks like a cramp or maybe a mild strain. She should be okay. I mean most of her shots were looking really good in that match and she was serving great, it was really just a conditioning and strength issue, which is fixable. Against a lot of opponents that's over in 2 sets pretty comfortably but Sakkari was just the best version of herself and the shape she's in is obviously an advantage.
It's very interesting comparing the Brooksby match to the Bianca match that followed. Bianca and Novak are obviously the more talented players in those matches, but when Sakkari manages to fight through a tough tiebreak after losing multiple set points and fighting off a bunch of break chances throughout the set, she knows it only gets easier going forward. When Brooksby plays a 20 minute service game and finally manages to grind out a break, Djokovic is just...
The mountain that still lies in front of you after exerting all that effort is I think what took the wind out of him. "I have to do that 16 more times" cannot be an easy thought to have run through your head.