Opening batsman Williamson produced a sensational batting show as he carried his bat through the innings with a 49-ball 101 not out, the first hundred of this edition, to guide his side to a huge total of 206 for five after being put into bat.
Chasing a huge target of 207 for a win, the Cobras struggled to get going and they were 44 for two when rain arrived at the Shaheed Veer Narayan Singh International Stadium.
As five overs had been completed before the rain came, the match was counted as complete and the par score under D/L method at that stage was 77.
The Knights kept their winning momentum in the tournament as they had won all their three qualifying round matches.
The Cobras run chase began on a disastrous note with their opening batsman Stiaan van Zyl (0) dismissed in the first delivery of the innings bowled by Trent Boult.
Robin Peterson and Omphile Ramela were batting on 17 and four respectively when rain came in the eighth over, though the Cobras were looking down the barrel at that stage as the asking rate had risen over 12 an over.
