Finding themselves ahead within the first five minutes, Cambridge United defended with splendid resolution, leaving the Owls with a tricky task if they are to progress beyond next Wednesday 's return leg at the Abbey Stadium.
Wednesday took the occasion seriously enough, fielding a full strength Premiership line-up, but any thoughts that they might be in for an easy night vanished in the fourth minute.
This was how long it took for the accident-prone Pressman to hand Cambridge what proved to be a gift-wrapped winning goal in one of those nightmare moments every goalkeeper dreads, or at least has done since the revision of the back-pass rule.
Juan Cobian played the ball back to Pressman, whose efforts to clear with his favoured left foot allowed Trevor Benjamin, the 19-year-old Cambridge forward, to be on him in a flash. He whipped away the ball and ran it into an unguarded net, much to the delight of the noisy Cambridge contingent in a sparse crowd of less than 9,000.
With something to defend, Cambridge pulled men behind the ball eagerly and heroically kept their opponents at bay, and Wednesday could not complain at a roasting from their manager Danny Wilson.
Cambridge worked tirelessly and earned some luck. Booth seemed sure to turn in Paolo di Canio's 57th-minute cross only for Jamie Campbell to get in the way. It was all deeply frustrating to Wilson, whose memories of Cambridge are not happy ones - he was a member of the Wednesday team beaten 4-0 at the Abbey Stadium in an FA Cup upset in 1991.
Wilson described Wednesday's performance as "bordering on total complacency" and accused some players of being self-indulgent.
"Too many of our players were playing for themselves and in games like this you only win by giving total commitment," he added.
Sheffield Wednesday (4-4-2): Pressman; Cobian, Thome, Walker, Hinchcliffe; Di Canio, Atherton, Jonk, Rudi (Whittingham, 67); Booth (Sanetti, 67), Carbone. Substitutes not used: Clarke, Hyde, Barrett.
Cambridge United (4-4-2): Van Heusden; Chenery, Duncan, Joseph, Campbell; Taylor, Russell, Wanless, Mustoe; Butler, Benjamin. Substitutes not used: Preece, Marshall, Youngs, Wilde, McCammon.
Referee: T Heilbron (Newton Aycliffe).