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 should be done away with since EVERYONE breaks it anyway.
"The real fault was the rule," Evert said at the Buoniconti Fund’s 33rd Annual Great Sports Legends Dinner.
"Everybody coaches on the side. Every coach coaches. ... So, change that rule."
As for Serena's legacy, Evert thinks "this is a blip on the whole picture of what she's done for women. And she's done so much more ... I think we should let this go."