Serena Williams wins again -- in a big way -- stripping down to her pregnant birthday suit for the cover of Vanity Fair ... and she SLAYYYYYYED.

In the issue, the tennis superstar also details how she found out she was pregnant on the set of a lingerie shoot (she took 6 pregnancy tests!) and how she fell in love with her fiance, Alexis Ohanian.

Also, Serena revealed she knew she was pregnant when she decided to compete in the Australian Open back in January ... a tournament she dominated and won!

In fact, Serena says she expects to return to pro tennis as soon as January.

Until then ... that cover. Damn.