With a supreme all-round display, as she also snaffled three wickets with her spin, the player-of-the-match heroics from 'JJ' also finally gifted the struggling Welsh Fire their first win of the campaign and ended the hopes of Ellyse Perry's Birmingham Phoenix to reach the playoffs.
Fellow Aussie star Perry finally found some form after a disappointing campaign by her lofty standards as she fired her first half-century of the tournament, but it was nowhere near enough to eclipse the brilliance of 32-year-old Jonassen, whose 44 off 17 deliveries and 3-24 off 20 balls set up the Fire's 36-run win.
Jonassen has lost her place in the international team but has shown in the Fire's last two matches just why she's still such a force in the women's game with her big-hitting and crafty bowling.
Her big moment came with 10 balls left of the Fire innings when she took a fancy to the spin of Hannah Baker.
After Hayley Matthews (34no) had taken a single off the first of the five-ball set, the Australian clouted the next four for sixes, the first two of which saw her nearly caught on the boundary.
The 25 runs off Baker's set was a record in the women's Hundred.
"Can't say I'd be on the top of anyone's list to achieve that," smiled Jonassen of her landmark feat, though those who watched her clean-hitting for Brisbane Heat in their WBBL final near-miss might beg to differ.
"Not too bad to be fair. I got a little bit of luck with the bat, kind of muscled a few, but I'm really pleased we finally got some points on the board," she added, reflecting on her narrow escapes.
"I just really wanted to take the game on particularly in those last 10 balls -- and it doesn't matter how far over the rope they go!"
Another Australian international Megan Schutt finally got her out but Jonassen's late impetus, with three fours to go with her quartet of sixes, dragged the Fire to 3-150 after Sophia Dunkley had kicked off their innings with a half-century.
Perry's Phoenix had to win to keep their tournament hopes alive but Jonassen struck the key blow to bamboozle and bowl her charging compatriot Georgia Voll after she'd raced to 29 off 18 balls.
Perry had to effectively play a lone hand, with her 55 off 43 balls, featuring seven fours and a six, still proving in vain.
The defeat sent Phoenix to the bottom of the table as the Fire leapfrogged them. "She's been amazing the last couple of games - with 4-10 in her last game nearly dragging us back into it, and now such a great show of hitting," said Fire captain Tammy Beaumont of Jonassen's display.
