In a rain affected match at Canberra's Manuka Oval, the Sixers won the first Sydney Smash of BBL|10 with eight balls to spare.
Chasing 129 to win from 14 overs under the Duckworth Lewis Stern method, due to a rain delay after Thunder had scored 6/166 from their 20, Josh Philippe (64 from 36 balls) steered the Sixers to the win with 5/132.
Alex Hales (54 from 33) and Daniel Sams (50 not out from 37) were the two Thunder batsmen to reach 50.
Sams returned for his first match since the concussion he suffered against the Brisbane Heat.
Hales hit five fours and three sixes in his innings before he became the second wicket for spinner Steve O'Keefe (3-15), who returned his best BBL figures.
The English opener has scored 323 runs in 10 games this season and is the second leading run-scorer in BBL|10 behind Philippe.
Sams guided Thunder past 150 hitting four fours and one six. He also took 2-25 with the ball.
Thunder's other wicket takers were leg-spinner Tanveer Sangha (1-14), Ben Cutting (1-17) and fellow pace bowler Nathan McAndrew (1-35).
Thunder will remain in Canberra for their next clash against the Hobart Hurricanes at Manuka Oval on Monday. Live on Seven, Fox Cricket, Kayo Sports and SEN radio.
Mon 18 Jan | Vs Hobart Hurricanes | Manuka Oval | 7:15pm