Rivers Angels had the upper hand in the opening exchanges. After winning the their third corner kick in the seventh minute, Mamuzo Edafe's delivery was goalbound and forced Asa Rabolao into tipping the ball over the bar.
Sundowns responded when Chuene Morifi dribbled past a couple of defenders and crossed the ball to Andisiwe Mgcoyi, who had a swarm of defenders around but she laid it back to Rhoda Mulaudzi, whose powerful effort went over the bar.
The Brazilians then won a corner in the 17th minute. Lerato Kgasago whipped it in and captain Zanele Nhlapho headed into the back of the net.
Mulaudzi then crossed for Mgcoyi, who glanced her effort over the bar. Kgasago then shot over the bar as Sundowns continued to ramp up the pressure.
The second half was less eventful, but Downs were keeping their shape well at the back while Rabolao was solid in goal.
Angels were coming back into it, though, and Juliet Sunday put a cross into the box but Vivian Ikechukwu's header was off target.
Due to their resolute defending, the Sundowns ladies withstood the pressure from Angels and closed out the win.
Meanwhile, Kenyan outfit Vihiga Queens moved into second in the group after a 2-1 win over Moroccan side FR Rabat, who dropped from second to third, while Angels are bottom.