Watching Djokovic has become a live-action demonstration of a tennis player figuring out his limits from match to match and week to week.

Novak Djokovic said that she was surprised to see Serena Williams watching him from the stands during the pre-quarterfinal against Lorenzo Musetti on Tuesday.

If the match continues to be lopsided, the main highlight will be Djokovic's heated debate with chair umpire Fergus Murphy. Thankfully, the Tennis Channel shared an up-close video of the exchange on social media.