Serena Williams -- Warned by Friends ... Don't Let Drake Play You Again!

Drake is getting a second bite of the apple that is Serena Williams -- but now that they're back on and popping, Serena's guard is up for one reason ... best summed up in one word: THOTs.

Sources close to Serena tell us she's been into Drizzy ever since they first hooked up in 2011, but that ended badly because he had his eyes on a few other chicks at the same time. Let's face it, Drake's game with strippers and Instagram models is legendary.

We're told Serena's willing to give him another chance because they have a "good vibe" -- which was on full display in the pics TMZ posted of their dinner in Cincinnati. Still, friends are warning her to be careful she doesn't get hurt again.

Our Drake sources say it's different this time -- he treats her with more respect than his other dates because Serena's a superstar in her own right. Translation: He doesn't have to worry about her using him for fame.

We're told she digs him showing up at her matches, and holds her family section tix for him. They're trying to take it slowly, but it's obvious from pics and videos ... they're crushing hard. Again.