Jozi finished second with 29 points, while Paarl ended in third spot on 22 points and now bow out of the competition.
Morkel slammed four fours and five maximums in his 57 not out off 28 balls.
The Paarl side, who got to their target in 15.2 overs, will now go on to face the Jozi Stars in the Qualifier on Friday.
Rassie van der Dussen and Dan Christian produced a 118-run partnership which powered the Stars to 239/3.
It was only a second win of the campaign for them. Though, they remained rooted at the end of the points table.
In reply, a 120-run stand between Du Plessis, who made 61 off 34 balls, took them too far ahead for the Stars
Cloete struck 65 from 59 balls which laid the foundation for the Spartans’ 165/4 in their total of 20 overs.
The Proteas opener smashed three sixes and 12 fours in his 47-ball cameo as the hosts chased down a 145-victory target with 4.3 overs to spare.
Two consecutive sixes in the final over from Jozi Stars’ Nono Pongolo saw his side to a one wicket win over the Durban Heat
Fawad Alam in line for a Test comeback?
Sindh claim victory in last-over thriller after Babar Azam ton
Miandad feels PCB has burdened Misbah
'Serious eye-opener for our system' - Misbah reflects on T20I series whitewash
Sourav Ganguly likely to become the new BCCI president
Langer: 'Smith might not want Test captaincy back'
Yadav, Jadeja inspire India to thumping innings win at Pune
Sarfaraz, Rizwan urge fans to throng the Iqbal Stadium
World Cup and Ashes summer made me even more hungry: Dom Sibley
WATCH: Azhar Ali, Asad Shafiq involved in bizarre run out