Surprise Moriri's 24th minute goal proved decisive at Loftus Stadium as the Tshwane giants extended their winning streak to seven games in the League. On the same night, Chiefs, who have a game in hand on Pitso Mosimane's outfit, suffered a 1-0 home loss to Mpumalanga Black Aces.
The opening exchanges were combative in nature, with time and space on the ball hard to come by on a greasy surface.
Downs attacker Khama Billiat nearly found a way through in the 14th minute, but his low shot was well kept out by Patrick Tignyemb following a fantastic through-ball from Man of the Match Teko Modise.
The Brazilians, though, took the lead 10 minutes later, as veteran midfielder Moriri headed home an inch-perfect free-kick by Modise from the right. The setpiece was harshly awarded for a slight push by Musa Bilankulu on Anthony Laffor.
Celtic had a good chance in the 35th minute, but on-loan defender Bilankulu glanced his header wide off a right-wing cross.
The second period was similarly cagey, with Laffor forcing a neat save by Tingyemb with a header, while Celtic substitute Frank Mhango dragged his weak shot wide moments later around the hour-mark.
Mhango, who came on for Magogi Gabonamong straight after the recess, tried his luck again from range in the 72nd minute but his attempt was well off target.
Two minutes later Modise sustained an ankle injury due to a bad tackle from Tsietsi Mahoa, and was replaced by Esrom Nyandoro, with Mahoa receiving a booking.
Phunya Sele Sele almost snatched an equaliser two minutes before the end but Joel Mogorosi headed wide off a route-one ball with Kennedy Mweene caught in 'no man's land'.