The holders, Manchester Giants, took their place in the last four with a fourth-quarter dash for the line after a three-pointer from James Hamilton gave Chester Jets a 69-57 lead.
The Giants' leading scorer, Tony Dorsey, who was denied the ball for most of the first three quarters, hit 13 points in the fourth, including the free throws that gave Giants the lead at 78-77. He also hit the three- pointer that stretched their advantage to 81-77 on the way to an 83-82 win.
The Giants' coach, Nick Nurse, said: "We were lucky and I'm not sure the best team won today. We needed to hit those three pointers in the fourth quarter to pump us up."
Thames Valley Tigers, who have won the trophy a record four times, outscored London Leopards 14-4 over the last four minutes for a 91-84 win, with Tony Christie scoring nine of their points in the final spurt.
London Towers also sunk their opponents with a decisive late burst as Danny Lewis and Steve Bucknall engineered a 13-8 run for a 76-72 victory over Derby Storm, despite two late three-pointers from Cypheus Bunton.
In the semi-finals at the NEC on 22 January, Towers play Tigers and Giants face the Sharks two weeks after they meet in the National Cup final.Reuse content