SUN CITY, South Africa, Nov 29, 2012 (AFP) - The pair finished with a two-under-par total of 70 at the 7162-metre Gary Player Country Club course in the northwest tourism resort of Sun City.
English defending champion Lee Westwood, South African hope Louis Oosthuizen and Paul Lawrie from Scotland were trailing by one shot.
Haas's father, veteran player Jay Haas, takes part in the seniors event of the tournament, but grabbed a moment to watch his son birdie the par-three seventh.
Haas junior was leading by two strokes when he bogeyed the par-three 16th and par-four the 18th.
"I got a little ahead of myself and made a couple of bogeys coming in," he said afterwards, disappointed with his finish.
This allowed Colsaerts, who at number 36 is placed just above Haas on the Official World Golf Rankings, to end the opening day tied for the lead with the American.
"I was in control of my ball all day," said a satisfied Colsaerts.
Sweden's Peter Hanson and Carl Pettersson, Martin Kaymer from Germany, Italy's Francesco Molinari, and South African Charl Schwartzel all finished the day on level par 72.
World number four Justin Rose of England, the highest-ranked player in the 'African Major', finished second last of the 12 players with one-over-par 73.
South Africa's Garth Mulroy carded three-over-par 75.
Scores after the first round of the Nedbank Golf Challenge invitation tournament at the par-72, 7162-metre Gary Player Country Club on Thursday:
70 - Nicolas Colsaerts (BEL), Bill Haas (USA)
71 - Paul Lawrie (SCO), Louis Oosthuizen (RSA), Lee Westwood (ENG)
72 - Peter Hanson (SWE), Martin Kaymer (GER), Francesco Molinari (ITA), Carl Pettersson (SWE), Charl Schwartzel (RSA)
73 - Justin Rose (ENG)
75 - Garth Mulroy (RSA)