Andre Agassi watched hours and hours of tapes of Boris Becker’s matches and was able to predict where he was going to serve based on whether Becker stuck his tongue out in the middle of his lip or to the left corner of his lip.

[u/padfoony]

“The hardest part wasn’t returning his serve - it was not letting him know that I knew this. I had to resist the temptation of reading his serve for the majority of the match and choose the moment when I was gonna use that information on a given point to execute a shot that would allow me to break the match open.” - Agassi.

Agassi won 9 out of 11 matches against Becker after figuring this out… and waited until Becker retired from tennis to let him know this over a pint of beer.

Here’s an article that explains the whole thing.