The close win ensured that the Cape franchise could not be overtaken at the summit of the table.
The Cobras have 33 points after nine matches, followed by the bizhub Highveld Lions on 26 points and the Chevrolet Knights on 22 points, also after nine clashes.
The Cobras managed 145 for four in their 20 overs. The Knights needed 18 runs to win off the final over, but Ferisco Adams produced a magnificent final over to limit the hosts to 137 for four.
The Cobras will host the Ram Slam T20 Challenge final on Friday 12th November at the Newlands Cricket Stadium against either the Knights or the Lions.
“We are very pleased we made it through to the Champions Trophy tournament again,” said Ontong.
The Cobras have some unfinished business in the global T20-showpiece, where they are still to win the title. They were disappointing in 2014 in India.
“Our spinners bowled exceptionally well,” the captain said about the superb performance against the Knights.
The 145 for four was a competitive score, but not necessarily a winning one.
But the left-arm spinner Robin Peterson produced a magical three for 21 in four overs, including the prized scalp of Rilee Rossouw. He also bowled 12 dot balls.
George Linde took one for 18, including the wicket of Reeza Hendricks. He has now taken 13 wickets in the domestic T20-competition at an economy-rate of 6.11.
“The secret was bowling a back-of-the-length delivery that either spin or skidded through. At the Wanderers it could be unforgiving, but in Bloemfontein there are big boundaries and the surface can support spin,” Peterson said.
Earlier, Richard Levi slammed 64 off 61 with two fours and two sixes on a surface that seemed to be slow and a bit two-paced following the rain in Bloemfontein on Friday.
“It’s probably the first and the last time I get as a batsman of the match award with a strike-rate of 110,” said Levi.
“But I will take the runs. Last season this time, I was going through a lean patch. I also did not play in the first match of the Champions League. It is nice to prove a point,” he added.
Levi has scored 369 runs at an average of 46.12 and a strike-rate of 146.42.