Nat Sciver's 46 led Surrey Stars to a four-wicket victory over Southern Vipers at Hove to leave her side in pole position to reach Finals Day.
Vipers captain Suzie Bates had anchored her team's innings with 82 off 57 balls but Danni Wyatt (15) and Mignon du Preez (14) were the only other two players to manage double figures as the visitors were set 148 to win.
England duo Sarah Taylor and Sciver shared a 73-run third-wicket stand for Surrey, only to fall before the end. But, requiring nine to win off the last over, Wyatt knocked over the stumps with her foot with the first ball to be called for a no-ball and Marizanne Kapp hit the resultant free hit for six to help see the Stars home with three balls to spare.
The Stars occupy the final place to reach Finals Day with 20 points, while Lancashire Thunder remain fourth on 17 points after their win over Yorkshire Diamonds.
After the Vipers were put in to bat, Bates struck 13 fours and a ramped six in her innings, but she proved to be the only player able to find runs on the pitch.
England opener Tammy Beaumont, who missed four matches in the tournament with concussion, was stumped by fellow international teammate Taylor for a golden duck, while Stars captain Sciver took 4-32 as the Vipers posted 147-9.
Despite missing main seam bowlers Tash Farrant and Katie George, who were ruled out for the rest of the competition with injury, 17-year-old Lauren Bell opened up with a maiden for the Vipers, picking up impressive figures of 0-16 from her four overs against a line-up that included Lizelle Lee, Taylor and Sciver.
New Zealand spinner Amelia Kerr (2-26) bowled Lee for 25 and had Taylor caught for 38 as Surrey slipped to 114-3 but Sciver batted exceptionally well to almost take her side home before she was run out four runs short of her half-century.
Bates was the one who ran Sciver out, and she also accounted for Dane van Niekerk in the penultimate over, returning figures of 2-22 in an impressive all-round day, but Surrey were still able to get over the line off Wyatt's last.
Meanwhile, in the Roses clash between Lancashire and Yorkshire, the Thunder eeked out a nine-run win to keep themselves in contention and eliminate their rivals.
England's Katherine Brunt again starred for the Diamonds, taking 3-22 to help restrict Lancs to 154-9 batting first, and then blasted an unbeaten 44 from 25 in the run chase, but it wasn't to be enough.
India's big-hitting batter Harmanpreet Kaur (74) was the Thunder's match-winner, cracking four fours and six maximums in her thrilling 44-ball knock.