Grace, who had started the day with an impressive four stroke lead, finished with a two-over 75 that included five birdies, but also three bogeys and two doubles.
That gave Colsaerts the chance to move level with the South African at the top of the leaderboard. The 29-year-old Belgian posted a 69 after he eagled the last from 50 feet, a remarkable comeback considering his uninspiring 76 on Friday.
In joint third are Retief Goosen (70) and Charl Schwartzel (68), while European Captain José María Olazábal carded an eight-under-par 72 for fifth.
Ireland's Padraig Harrington and 2010 Open champion Louis Oosthuizen share sixth on seven-under-par, while Ernie Els is a further stroke back on six-under-par.
