Serena Williams -- Hangin' with Drake's Mom ... At Restaurant Opening

Big step for Serena and Drake ... the tennis star jetted up to Canada for the opening of Drake's restaurant Monday night -- and hung out with the rapper's mother!!

Drake was in Toronto to launch his new eatery Fring's ("I got onion rings, gotta lotta onion rings") ... along with the two ladies in his life.

Serena looked smokin' hot -- high heels and a pencil skirt -- and according to people inside, there was the usual restaurant PDA with Drake.

Unclear how much time Serena spent with Drake's mom -- but we're told the music was so loud inside, it would have been tough to have that "So, you're dating my son" conversation anyway.

As for Drake, he posted a pic from inside the restaurant with his mom ... calling her his "one and only."

No word if Drake's dad showed up -- after we spoke with him last time, seems he was also waiting for some quality time with his son's new boo.