Two strokes clear with four holes remaining when the tournament was forced into a fifth day because of a thunderstorm, Els limped home with four successive pars at Bay Hill to hold off his challengers.
The smooth-swinging South African, who this month claimed his first tour victory in two years at the elite WGC-CA Championship, closed with a one-under-par 71 for an 11-under total of 277.
"It was a hard struggle today," Els told reporters after sinking an eight-foot par putt at the 18th to secure the winner's check for $1.08 million. "I really earned this one ... and two wins (this season) is definitely special.
"It was a really great test for the mind the last 24 hours, it was really pretty tough for me. I'm a guy who puts a lot of pressure on myself ... but I've always been like that.
After early morning rain had delayed the resumption of the final round by two hours, three-times major winner Els did well to par his first hole, the 15th, after missing the green to the left with his approach.
The 40-year-old also found himself in trouble at the par-five 16th where he found the right rough off the tee and had to punch out a low second shot from under trees on to the fairway.
From there, he struck his third to 30 feet and coolly two-putted to save par.
Els got up and down from a buried lie in a greenside bunker at the par-three 17th to stay one stroke in front, until Na bogeyed the par-four last after driving into rough to slip two behind.
With the tournament firmly in his grasp, Els struck a perfect tee shot down the 18th fairway but he pulled his approach left into greenside rough from where he again nervelessly got up and down to save par.
"Just to try and stay away from a lot of trouble here was tough to do," said Els, who was also champion here in 1998.
"This place tests your ability and it (winning) gives me a lot of confidence. If you win here, you can feel comfortable at majors.
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Ed Osmond and Sonia Oxley)