Ndumiso Mabena almost opened the scoring in the eighth minute with a well timed low shot inside the box that was saved by Reyaad Pieterse.
Siwelele were ahead in the 15th minute as Mabena played in Victor Letsoalo, who rounded Pieterse to score into an empty net.
Thuso Phala could have equalised in the 29th minute with a thumping shot from just outside the box, but Patrick Tignyemb made a great reflex save.
Matsatsantsa were cut open on numerous occasions, with the Celtic trio of Kabelo Mahlasela, Letsoalo and Mabena threatening to run riot.
The home side recovered five minutes before halftime but it was too late to gather any momentum and they went into the break trailing by one goal.
Five minutes after the break, Mahlasela let fly with a left-footed strike but Pieterse collected comfortably at his bottom right corner.
Veselin Jelusic's charges were all over United in the opening stages of the second half as they looked for a second goal.
Eric Tinkler made his first alteration two minutes before the hour by bringing on Siyabonga Nhlapho in place of Reneilwe Letsholonyane.
Jeremy Brockie had a chance to restore parity in the 72nd minute with a header inside the box but his effort sailed agonisingly wide.
Three minutes later, Darren Smith levelled matters for United with a header at the far post following a cross from Aubrey Modiba, 1-1.
The game tightened up with five minutes left to play as both teams were cautious to commit numbers upfront.
Mpho Maruping nearly snatched the winner for Celtic two minutes before time but his header sailed over the crossbar as the game ended 1-1 on the night.