In contrast, McIlroy's 74 included four bogeys and a double bogey, while Stenson - who has finished in the top 15 in his last five appearances in the event - double-bogeyed the 18th in a 75 matched by Blayne Barber.
The American was a last-minute replacement for Danny Willett after the Masters champion withdrew due to illness.
Fitzpatrick eclipsed Nick Faldo's record as the youngest Englishman to win three times on the European Tour with victory - and the first prize of more than £1million - in the DP World Tour Championship in November, two years to the day after gaining his card via the qualifying school.
The 22-year-old from Sheffield has taken advice from McIlroy on buying property in Florida as he plans to gain his PGA Tour card, a task which would be completed with a win at Bay Hill.