Thunder 124 for 5 (Boyce 31) tied with Sunrisers 124 for 4 (Griffith 63, Cross 2-20, Jones 1-20)
Kate Cross and Hannah Jones's death bowling helped Thunder pull off a thrilling tie with Sunrisers after Cordelia Griffith had swiped her maiden Charlotte Edwards Cup fifty.
Griffith had put Sunrisers on course for victory with an outstanding 63 in their run-chase of 125. But with nine needed from the last two overs, England fast bowler Cross bowled Griffith and only conceded four runs.
Spinner Jones had Florence Miller caught off the fourth ball of the final over before Jo Gardner could only scuff a single from the last ball to give the competition its first tie.
It means Sunrisers can no longer qualify for Finals Day next Sunday, while Thunder can still top Group B and reach the Ageas Bowl showpiece if they beat neighbours Northern Diamonds on Monday and Western Storm lose.
Griffith was forced to be patient against an exemplary Powerplay from the Thunder's bowling attack - with only 23 runs coming off a slow pitch and Naomi Dattani departing. She soon caught up though with a four and a six with two swings down the ground in the seventh over, before another lusty blow over long-on gave the run-rate a much-needed boost.
A third six, this time dispatching a full toss over midwicket, followed quickly by a cut to the boundary brought up the half-century partnership with Grace Scrivens - Griffith had dominated with 38 of the runs. Her own fifty came up with a worked single into the leg side in 40 balls.
Scrivens joined in the fun with a pulled maximum off Cross but was caught on the long-on boundary to end a 79-run stand, with 23 runs still required.
Griffith made the hosts favourites again with two fours through the covers but Cross and Jones' heroics meant the two sides shared the points.
Having been put in, Thunder's innings was made up of two substantial partnerships - Georgie Boyce and Daisy Mullan's 51-run opening stand and 49 put on by Ellie Threlkeld and Cross - punctuated by spells of accurate and tight bowling.
Boyce and Mullan - standing in for Emma Lamb, who had been a late call-up for England, for just her third appearance - made things look easy with 48 runs off the Powerplay.
But the good start faltered as Boyce picked out long-on and Emily Thorpe entered the attack to have Mullan caught at mid-on and Piepa Cleary mistiming to midwicket - the leg-spinner claiming 2 for 17.
Cross and Threlkeld rebuilt again with both striking three fours, and the former clubbing a six straight back down the ground. However, both were bowled in the dying embers of the innings.