Serena Williams -- CHECK OUT MY $75K WATCH ... 323 Diamonds!!

Serena William's swag just reached new levels -- 'cause the tennis superstar got herself a new luxury watch that comes fully equipped with 323 DIAMONDS!!!

It's called an Audemars Piguet Millenary timepiece -- made with 18-carat pink gold, a black mother-of-pearl dial and 323 brilliant-cut diamonds! The watch usually runs around $75K.

A rep for Audemars -- popular with celebs like Jay-Z -- says the luxury watchmaker has a relationship with the tennis champ ... but this watch wasn't a freebie from them.

"Serena is a friend of the brand but that isn't a piece we gave her," the rep tells TMZ Sports.