Serena gave birth to little Alexis Jr. on Sept. 1 -- and spent several days in the hospital due to "complications."

But early Thursday morning, Serena posted her first full-body post-preggo selfie -- and she looks superhero fit.

Seriously, this is a woman who won a major tennis tourney while pregnant -- who crushed it in the gym at 35 weeks pregnant -- and whose mother guaranteed she'd return to dominate the pro game after the kid was born.

So, when will Serena return? The next Grand Slam is the Australian Open in January ... the same tourney she won while she was in her 1st trimester.

You've been warned.