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 .