Serena Williams On No Donut Diet

It takes great sacrifice to be a great champion and Serena Williams is making the biggest sacrifice of all -- DONUTS!!!!

The tennis superstar was greeted at LAX by a fan who wanted to gift Serena with a dozen delicious treats from the legendary Randy's Donuts in L.A.

Problem is, Serena had to shoot down the offer explaining, "I'm not eating donuts right now. I'm trying to get fit."

We get it ... she's gearing up for her return to the court at the ASB Classic in Auckland, New Zealand in January.

But for the record, she looks pretty damn fit to us.