Tunnicliff, who opened the round with five consecutive birdies, shot a 4-under-par 69 and is at 10-under 136 after 36 holes in the European Tour event. Markus Brier had a 7-under 66 and Branden Grace matched Tunnicliff's 69 and are tied for the lead.
Ross McGowan (69) , Louis Oosthuizen (67) and Jaco Van Zyl (70) are tied for fourth, one stroke off the lead and Elliot Saltman (71) is alone in seventh at 8-under.
First-round leader Brandon Pieters turned in a 4-over 77 and slipped to a tie for 26th, seven strokes behind the leaders.
Tunnicliff started his second round with five birdies but gave strokes back at the sixth and ninth holes. He played the back nine of the East London Golf Club in 1 under with three more birdies, including the 18th-hole chip-in, and two bogeys.
Brier, who had an eagle, eight birdies and three bogeys, pulled into the lead with consecutive birdies on Nos. 6-8 but bogeyed the ninth hole -- his final of the day.
Grace's round featured six birdies -- four in the span of Nos. 3-7 -- and two bogeys.