Jonson Clarke-Harris took his tally to 29 goals for the season after heading in Peterborough's fourthPeterborough United belied a 19-point regular-season gap to take a commanding 4-0 first-leg lead over Sheffield Wednesday in their League One play-off semi-final.
Jack Taylor's early opener put the hosts on the front foot before Joe Ward's long-distance second flew into the top corner after clipping Marvin Johnson.
Kwame Poku headed Posh's third soon after the restart as Wednesday contemplate play-off defeat for a second successive season.
It got worse for the visitors late on as Jonson Clarke-Harris nodded in a fourth from close range.
The Owls had a number of good chances throughout the game but a combination of good goalkeeping by Will Norris, last-ditch defending and poor finishing leaves Darren Moore's side with a herculean task in Thursday's second leg if they are to make the final at Wembley later this month.
The Owls came into the tie on a four-game winning streak, having finished the regular season on 96 points - the highest-ever tally by a side not to have won automatic promotion in the EFL.
Meanwhile the hosts only made the top six thanks to Wednesday's final day win over Derby, which allowed them to sneak in after a win over fourth-placed Barnsley under returning manager Darren Ferguson.
Wednesday should have been ahead inside seven minutes when Ollie Norburn was dispossessed by Michael Smith on the edge of the centre circle, allowing the striker to bear on goal, but his low shot from the edge of the box was palmed away by Will Norris.
Having survived that mistake it was two errors that led to Peterborough's opener - Will Vaulks' poor clearance from a cross fell to Taylor, whose powerful low shot from 15 yards squirmed through Owls keeper Cameron Dawson when he should have saved it.
Wednesday almost levelled three minutes later when Callum Paterson's goalbound volley from a corner smashed against his team-mate Windass, as both sides tried to assert their control on the tie.
It was the hosts who did take control and it was a good spell with the ball that saw Ward unleash a powerful strike from 25 yards, that spun off Johnson and spectacularly into the top corner after clipping the underside of the bar.
Norris produced another fine save from Windass from the corner of the six-yard box soon after, having seen the striker get the wrong side of his marker thanks to a precise Michael Ihiekwe through ball.
And the Owls should have pulled a goal back four minutes after the restart when Barry Bannan's wonderfully-floated free-kick was headed wide at the back post by Ihiekwe.
Sheffield Wednesday seem set for a third season in the third tier after this heavy defeat - they were beaten at the same stage by Sunderland 12 months agoBut a minute later it seemed as though Wednesday's hopes were over - Ephron Mason-Clark's cross from the left found an unmarked Poku and his powerful header ended a fine counter-attacking move.
Peterborough top scorner Clarke-Harris headed a free-kick just over the bar soon after as Posh took control, but with 20 minutes left the Owls thought they had a way back.
First Johnson's shot hit Ronnie Edwards' arm on the line, but the referee did not see it - a decision you felt would have been overturned had there been VAR at the Weston Homes Stadium - before two Ihiekwe efforts were cleared off the line.
Any hopes that Wednesday might have of heading to Wembley were all but extinguished with nine minutes to go - Dawson acrobatically pushed a Taylor shot from close range on to the post, but the scorer of the opening goal did well to recover and clip a cross to Clarke-Harris, who nodded home from a couple of yards at the far post.
It could have been 5-0 had Dawson not saved well from Poku from inside the area in the sixth minute of stoppage time.
Line-ups
Peterborough
Formation 4-2-3-1
1Norris
23Ward6Kent4Edwards16Burrows
22Kyprianou18Norburn
11Poku8Taylor10Mason-Clark
9Clarke-Harris
1Norris23Ward6KentBooked at 15mins4Edwards16Burrows22Kyprianou18Norburn11PokuBooked at 64mins8Taylor10Mason-ClarkSubstituted forJonesat 90+4'minutes9Clarke-HarrisSubstitutes
3Butler5Knight17Jones24Thompson26Randall28Blackmore39TshimangaSheff Wed
Formation 3-4-1-2
25Dawson
6Iorfa44Flint20Ihiekwe
2Palmer4Vaulks10Bannan18Johnson
13Paterson
11Windass24Smith
25Dawson6Iorfa44FlintBooked at 54mins20IhiekweBooked at 55minsSubstituted forJamesat 86'minutes2Palmer4Vaulks10Bannan18Johnson13PatersonSubstituted forGregoryat 59'minutes11Windass24SmithSubstituted forAdeniranat 62'minutesSubstitutes
8Adeniran9Gregory17Dele-Bashiru19Bakinson31Stockdale32Hunt33JamesReferee:Anthony BackhouseAttendance:12,569Live Text
Match ends, Peterborough United 4, Sheffield Wednesday 0.
Second Half ends, Peterborough United 4, Sheffield Wednesday 0.
Attempt missed. Hector Kyprianou (Peterborough United) right footed shot from outside the box is too high.
Attempt saved. Kwame Poku (Peterborough United) left footed shot from the centre of the box is saved in the centre of the goal. Assisted by Harrison Burrows.
Foul by Jonson Clarke-Harris (Peterborough United).
Dominic Iorfa (Sheffield Wednesday) wins a free kick in the defensive half.
Substitution, Peterborough United. Ricky-Jade Jones replaces Ephron Mason-Clark.
Oliver Norburn (Peterborough United) wins a free kick in the defensive half.
Foul by Lee Gregory (Sheffield Wednesday).
Attempt missed. Dennis Adeniran (Sheffield Wednesday) header from the centre of the box misses to the left.
Joe Ward (Peterborough United) wins a free kick on the right wing.
Foul by Marvin Johnson (Sheffield Wednesday).
Substitution, Sheffield Wednesday. Reece James replaces Michael Ihiekwe.
Attempt saved. Michael Ihiekwe (Sheffield Wednesday) header from the centre of the box is saved in the top right corner.
Corner, Sheffield Wednesday. Conceded by Jonson Clarke-Harris.
Goal! Peterborough United 4, Sheffield Wednesday 0. Jonson Clarke-Harris (Peterborough United) header from very close range to the bottom left corner. Assisted by Jack Taylor.
Attempt saved. Jack Taylor (Peterborough United) right footed shot from the centre of the box is saved in the bottom right corner. Assisted by Joe Ward with a cross.
Ephron Mason-Clark (Peterborough United) wins a free kick on the left wing.
Foul by Liam Palmer (Sheffield Wednesday).
Ephron Mason-Clark (Peterborough United) wins a free kick in the defensive half.
