After her more celebrated Australian teammates had missed the boat and a straightforward-looking run chase turned a bit complicated against Sophie Molineux's Southern Brave, the ever reliable 33-year-old guided Leeds home by five wickets alongside Indian star Deepti Sharma.
Their victory in Friday's 'eliminator' set up the champions, who've been rebranded as the Sunrisers after winning as Northern Superchargers last year, to face Ash Gardner's Trent Rockets in the final at Lord's on Sunday (Monday AEST).
The Sunrisers had been reliant on the tournament's two leading run scorers Phoebe Litchfield and Annabel Sutherland in their recent recent run of stellar form but, chasing the Southampton-based Brave's paltry 7-114, on-song opener Litchfield was bowled leg-stump by Izzy Wong for just four.
When Sutherland then fell to a fine leaping catch by Molineux off Sarah Glenn for a quickfire 17 off 12, leaving Leeds at 3-54, it needed Sharma and Lauren Winfield-Hill to steady the ship.
But Winfield-Hill (37) and captain Dani Gibson, golden duck, fell to Wong in successive balls, requiring alrounder Jonassen, who's had a fine season in Yorkshire, to call on all her experience, hitting an unbeaten 10 off five balls, including the thumping winning boundary to the cover fence.
With Sharma having been every bit as good, slamming 28no off 22, the Sunrisers got to their target on 5-116 with 11 balls to spare, winning a fifth match in a row and likely to start as favourites against the Rockets, despite the Nottingham-based side having lost just once all tournament.
It was a disappointing end to a triumphant summer for Molineux, who had led Australia to T20 World Cup glory at Lord's and had hoped for another visit to the home of cricket.
She batted well but not quickly enough (22no off 24) after the Brave, put in by Leeds, struggled to break the shackles following Lizelle Lee's brisk early 32.
Another Australian Charli Knott hit 25 off 21, but Jonassen proved her worth the the ball too, giving away just 13 runs off her 15 wicketless deliveries.
Sutherland can rarely be kept out the headlines and she finished her 20-ball stint (2-31) by almost grabbing a hat-trick in her final five-ball 'over'.
Gibson dropped Wong in the deep - a tough chance - off her bowling after she'd dismissed Jodi Grewcock and Naomi Dattani with the previous two deliveries.
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