Davidse spent four days in hospital with a bad bout of flu after missing the cut in Mauritius last week, but carded nine birdies and a bogey to lead by a single stroke from a group of six players, including Englishmen James Morrison and Matthew Baldwin.
Another 10 players are a further shot back at six under-par on a good day of scoring on the Firethorn and Bushwillow courses that were used for the 240-strong field.
"I'm obviously very happy. I've worked very hard. I went to Mauritius last week and didn't play too well. I was in hospital for about four days so I'm very chuffed today,” Davidse told reporters.
“I didn't really have any expectations coming into the week. I'm just out here to have fun. Some guys say this golf course (Bushwillow) is easy, but you’ve still got to hit the ball. It’s a good test, you still need to hit it in the right areas and make the putts.”
The Joburg Open is the final tournament of the year on the European Tour, and is co-sanctioned by the Asian Tour and South Africa’s Sunshine Tour.
Davidse’s best ever finish on the European Tour was in this tournament 10 months ago when he finished tied for seventh.
(Reporting By Nick Said; Editing by Toby Davis)
