Coco Gauff's Coach Brad Gilbert Says He Celebrated US Open Win With Tequila

Coco Gauff isn't old enough to drink just yet, but her coach was able to down enough booze for the both of them after her huge US Open win last weekend ... with Brad Gilbert telling TMZ Sports he celebrated with "a little too much tequila."

The 19-year-old tennis superstar has a lot to celebrate after beating Aryna Sabalenka last Saturday ... becoming the youngest American woman to win the tournament since Serena Williams did at age 17 in 1999.

Gilbert -- who has coached big names like Andre Agassi and Andy Roddick -- raved about Coco's success at LAX earlier this week ... saying, "It was a great summer, an amazing summer."

"Coco is an incredible kid, she's humble, she's hard-working and her journey is just beginning."

Gilbert said Coco knows what it takes to continue going in the right direction ... adding, "She would tell you, it's just about getting better. Listen, everyone wants to do incredible things but she has a great opportunity in front of her to continue to do some great things."

"She's incredibly humble and she's poised, she speaks incredible for her age and her parents have done an amazing job. She's got a great team and just fun to let me be a part of it."

As for the post-match festivities, Gilbert said the law still applies for champions ... so Coco was downin' H2O while the adults let loose a bit.