Pietersen (67) reached his half century off just 27 balls and crunched 10 fours and a six as Stars chased down Thunder???s paltry total of 106-8 with 33 balls to spare at Sydney Showground Stadium.
The 34-year-old???s pyrotechnics lifted Stars up to the third in the standings and they will secure a last-four clash if they conquer defending champions Perth Scorchers in their final game on January 21 or even earlier if city rivals Renegades fail to beat table toppers Adelaide Strikers on Monday.
Pietersen, meanwhile, is now the top run scorer in Big Bash 04, having smoked 262 in his six innings to date at an average of 52.40.
Pietersen???s fielding was called into action at the start of Saturday???s game, with the former England batsman holding onto a low catch at mid-off to remove Thunder???s Daniel Hughes in the second over.
Surrey star Jason Roy had been dismissed for a first-ball duck before then and when Jacques Kallis edged behind to wicketkeeper Tom Triffitt in the fourth over, Thunder were reeling on 9-3.
Aiden Blizzard (45) and Andrew McDonald (23) put on 59 in 11 overs as Thunder looked to rebuild, but when McDonald was bowled looking for a big hit in the 15th over with the score on 68, the hosts could only muster a further 38 runs.
Michael Beer, Jackson Bird, John Hastings and Clint McKay each plucked two wickets for Stars, with spinner Beer and metronomic paceman Bird going for less the four runs an over.
England???s Luke Wright departed early when the Stars came out to bat, dismissed by Josh Lalor for four??in the??second over, but Pietersen swung freely from that point as the Melbourne side won with ease.
Pietersen, who scored 49 and 54 in his previous two games for the Stars, as well as 66 on debut in December, gave particular punishment to Nathan Hauritz, carting the twirler for three successive fours at the start of the 10th over.
Stars skipper Cameron White (23) supported his on-fire team-mate during a partnership of 76, but exited following some stunning fielding from Lalor in the deep, with the Thunder seamer throwing White???s skier back into the field of play before he tumbled over the rope and then scooting back to claim the catch.
However, Peter Handscomb (10) joined Pietersen at the crease and flicked Hauritz through midwicket for a single to wrap up the Stars??? fourth straight victory in the Big Bash following three losses to begin the competition.
Thunder are no longer in contention for a semi-final berth after suffering their fourth reverse??this term??but will close out their campain with a local derby against the Sydney Sixers on January 22.