The pair's battle to finish top of the PGA Tour money list stands to go down to the wire at the event.
Donald has been in pole position for the title for much of the season but Simpson looks likely to pass him this week following a fine first two days at the McGladrey Classic.
Donald isn't in action this week but has added next week's tournament to his schedule in a bid to ensure he finishes ahead of Simpson in the earnings list.
"It's going to be fun," said Simpson.
"He's one of the most competitive guys on tour and so I'm sure he's going to come guns loaded and he's going to play great like he has all year."
Simpson is currently two shots behind second rounder leader Billy Horschel at the McGladrey Classic as he attempts to secure the top-15 finish he needs to go top of the PGA Tour money list.
The American is currently $68,971 (£43,602) behind Donald.