Serena Williams won't just leave the US Open with a loss in the Championship ... she'll leave a little lighter in the wallet too.
Williams will be fined $17,000 for 3 separate code violations during the tennis tournament's final.
-$4,000 for a coaching violation.
-$3,000 for smashing her racket.
-$10,000 for verbal abuse towards the umpire.
As we reported -- Serena was penalized a game during Saturday's match against Naomi Osaka after the umpire accused her of receiving coaching from the sidelines -- calling him a thief for doing so.
Williams approached the umpire and heavily disputed the violation, saying she'd rather cheat than lose.