Joe Denly produced the best 29 balls by an Englishman with the bat all summer before Sydney Sixers captain Moises Henriques ended the Thunder's BBL finals hopes with a thrilling last-ball victory.
After the Thunder scrambled to 6-156 from their 20 overs, the Sixers appeared to have the match in the bag at 1-69 when Denly was bowled for 43 at the end of the ninth over at the SCG.
However, miserly four-over efforts from spinners Chris Green (0-22) and Fawad Ahmed (1-22) put the game back in the balance and left the Sixers nine to win from Green's last over.
But Henriques, on return from personal leave, saw the Sixers home, driving the final ball for the two required to ice the victory with eight wickets still in the shed.
It marked the fourth time this season a Sixers game had gone down to the last ball, but was their first win of the competition.
It helped the Sixers climb off the bottom of the ladder and above the Melbourne Stars but, more importantly, left the Thunder with next to no chance of qualifying for the finals with two games to play.
It also means the competition is theoretically a five-horse race with two weeks to play to finals. Hobart are the only team who can knock Perth, Adelaide, Brisbane or the Melbourne Renegades out of the playoffs.
"Pity for us," Green said.
"All season we've (fought back in games). We've had three tight losses which could have been three tight wins for us.
"It's a different story with three more wins and we're sitting neatly there in second or third on the table and guys like James (Vince) and Usman (Khawaja) back in our side we're preparing ourselves for finals cricket."
In his first game for the club, Denly hit four fours and cleared the rope three times while opening partner Daniel Hughes anchored the innings with an unbeaten 66 from 55 balls.
Denly, who played internationals for England in 2009 and 2010, only arrived in Australia a week-and-a-half ago but looked instantly at home after a stint in the Bangladesh T20 competition.
"I've had a busy winter playing a lot of cricket," he said.
"I've been able to keep that form going from England."
Earlier, Green put the Thunder back in the game with the bat with a quick-fire 49 off 27 balls as part of a 59-run stand with Arjun Nair (17).
After the Thunder went at a run a ball in the first 10 overs, Green ensured the visitors added 96 runs in the second half of their innings in front of a crowd of 36,458.