The Englishman posted a first-round score of 66 to end the day on six-under par in Florida. Simpson carded a 68 to put him on four-under.
The two players are both aiming to top the PGA Tour money list by the end of the tournament.
"Six birdies, no bogeys is always a good round," Donald said.
"I played a pretty solid round. Couple loose shots at the end. It wasn't that easy out there. The wind was strong enough to make it a little bit tricky with club selection."