Leopards stormed to a 32-14 lead at the end of the first quarter, led 52-34 at half time, and 66-56 after the third. The Sharks must now win both home games this week to reach the play-off finals at Wembley on the first weekend in May.
Fifth-placed Newcastle Eagles made home advantage count with a 78-76 win over London Towers, but Derby Storm's boisterous support failed to intimidate Thames Valley Tigers, who won 91-87.
Motivated by the mutual antagonism between the teams and coaches, the Tigers pulled away with a 10-5 finale to win, before retreating to their own intimidating Bracknall venue for Wednesday's second, and possibly decisive, game.
Last season Derby coach Bob Donewald, then at Leicester, accused Paul James of underhand tactics in securing the Tigers coaching post at the expense of Mick Bett. Earlier this season, one of Derby's repeated brushes with authority followed a game at Bracknell.
James said: "It is always very intense because of the history between the teams and between Bob and I. We went up there determined to stand our ground."
The decisive factor, however, came from what Tigers threw at Derby. Seven three-pointers from Casey Arena compensated for what James described as "poor perimeter play and a non-existent inside game" from his team.
James said: "A smaller, quicker line-up worked best. Lee Walker scored 16 and Stewart Clark scored 14 and took ten rebounds."
Yorick Williams, playing his first game since suspension following the brawl at Chester in January, scored 19 for Derby. Maurice Robinson scored 16 after being cleared to play by Friday's appeal against a one year ban for a doping offence in March.
Last season a three-pointer from London Towers' Danny Lewis knocked Newcastle out at the quarter final stage. Eagles' coach Craig Lynch held his breath again when Lewis launched a long range attempt to snatch victory at Newcastle.
A 14-1 run apparently gave Newcastle control at 45-44 and then, after London came back, Andrew Bailey and Ralph Blalock opened a 78-73 lead. Malcolm Leak's three-point play set up the chance for Lewis' match-winning attempt.
Eagles leapfrogged to fifth on the final weekend of the regular season, avoiding an anticipated quarter final with runners-up Manchester Giants to face instead fourth-placed London, who they swept 3-0 in the league.
"Now we've got to get on the road and do it again," said Lynch of their second leg on Thursday.Reuse content