Frost, a two-time winner on the European Tour and 10-time winner on the PGA Tour, also shot a 69 on Saturday to reach 7 under par for the championship.
Charl Schwartzel carded a 2-under 70 and is alone in third place at minus-6. Oliver Whiteley managed an even-par 72, but has sole possession of fourth place at 5-under-par 211.
Ernie Els, a three-time champion and world No. 3, shot a 2-under 70 and is back in the hunt after a second-round 75. Els, who owns a house on Leopard Creek, is tied for fifth place with Michael Kirk (69), Euan Little (72) and Warren Abery (73), who held the lead throughout the third round. The group is knotted at minus-4.
Cheetham moved up the leaderboard thanks to steady play on the front nine and his closest competitors making bogeys. Cheetham, who earned his 2005 European Tour card at Q School last month, tallied only one birdie on the first side, at the sixth, then made eight pars to move into the lead at 6 under par.
On the second nine, Cheetham held it together with six consecutive pars from the 10th. He moved one ahead of Frost with an 8-foot birdie putt at the par-5 15th, a hole Cheetham has birdied in all three rounds.
Frost got to 7 under, but Cheetham once again climbed past the 45-year-old international star. Cheetham's second shot at the par-5 closing hole landed 5 feet from the hole. He only managed a two-putt birdie, but he will be in the lead as he tries for European Tour victory No. 1.
Frost, a two-time Presidents Cup player from South Africa, collected a birdie at the second, but dropped a shot at the 11th. He rebounded with a birdie at the 12th, then matched Cheetham in the lead with a 25-foot birdie putt at the 13th.
Frost, who won this title in 1994, parred four in a row from the 14th, but picked up one more birdie on the way into the clubhouse. He got up and down for birdie from a greenside bunker at the 18th that momentarily gave him a share of the lead.
Even though Cheetham birdied the last for the 54-hole lead, Frost is now in good position to visit the winner's circle for the first time since the 1999 South African Open.
Richard Finch posted an even-par 72 and is alone in ninth place at minus-3. Mark Davis (72), Damien McGrane (72), Alessandro Tadini (73) and Lindani Ndwandwe (73) are tied for 10th place at 2-under-par 214.
Second-round leader Peter Gustafsson struggled on Saturday. He birdied the second, but bogeyed three in a row from the third. Things did not get better on the back nine as he got that nine-hole stretch started with three bogeys and a double bogey.
Gustafsson shot a 7-over-par 79 and is part of group tied for 19th at even-par 216. Defending champion Marcel Siem carded a 1-under 71 and is also part of that logjam.