Serena Williams got a "raw deal" at the U.S. Open -- all because of a DUMB rule that should be changed immediately ... so says tennis great Chris Evert.
The rule -- players are forbidden to receive any instructions from coaches on the sideline.
The ump at the U.S. Open claimed Serena and her coach violated the policy and he docked her a point penalty. Things went nuclear from there and Williams was later penalized a full game.
Enter Evert ... who tells TMZ Sports both sides deserved blame for the altercation -- but ultimately, it proves the anti-coaching rule is stupid and she be done away with since EVERYONE breaks it anyway.