Chasing 138 to win, Sydney Sixers overcame the challenge of impressive spinner Aaron Phangiso (3/14) to reach 141 for five in 19 overs at Newlands.
Electing to bowl, Sydney did well to stop Highveld Lions to a modest 137 for nine in the Group B match as Mitchell Starc took three wickets for just 19 runs.
Opener Gulam Bodi top scored for Lions with a 44-ball-61 while Jean Symes contributed 23 off 16 balls.
Except for Dominic Thornely (0) all top-order batsmen contributed in Sydney's win with skipper Brad Haddin scoring 32 after Watson laid the platform for the win.
Watson's 47 came off 38 balls with seven boundaries including three sixes. He strapping all-rounder will not be play for the side for the remainder of the tournament as he has been withdrawn by Cricket Australia, fearing burn-out.
Both the sides had two wins each before coming into this contest and with three wins in as many matches has ensured Sydney a place in the last-four stage of the tournament.
More From This Section
Lions, the local outfit, still have a match against qualifiers Yorkshire at their disposal on Saturday to claim the other semifinal berth from Group B.
Watson and Michael Lumb (22) gave Sydney a good start by putting on 60 for the first wicket after a slow start.