Downs had a sluggish start to the season after being booted out of the MTN8 and being held to a goalless draw against Platinum Stars in their league opener.
A brace from Cuthbert Malajila and goals by Khama Billiat and Teko Modise handed Sundowns maximum points.
Daine Klate scored twice for the Students.
“It was a difficult game against a good team,” conceded Mosimane, “they are title contenders and have a good coach. It is still too early, but it is an ice-breaker.
“I told the boys that if they want to win the championship, they must start winning games like this against credible opponents like Wits.
“Games like these are title-winning games,” added Mosimane.
