Kevin van Veen's second-half strike denied Pompey victory against promotion-chasing Scunthorpe.
Van Veen's 53rd-minute striker cancelled out Jamal Lowe's first goal since mid-September in the 1-1 draw at Fratton Park.
Kenny Jackett sprang a surprise with a switch to a 3-5-2 formation against their in-form opposition.
He was rewarded with an impressive first-half display full of attacking intent and ideas from the Blues.
Pompey couldn't maintain the intensity of their play, though, and found themselves under increasing pressure after the break.
The first chance of the game arrived for the visitors after two minutes, when Tom Hopper found space behind Pompey's back three and poked the ball past the advancing Luke McGee, but Jamal Lowe was there to clear off the line.
The breakthrough arrived after 15 minutes when Lowe bagged his fifth goal of the season, as he nicked the ball off Josh Morris before advancing and finishing from a tight angle.
Brett Pitman fired narrowly wide after 32 minutes as the Blues' confidence grew and the home crowd showed appreciation of their side's positive play.
The best the visitors had to offer was a 25-yard thump from Murray Wallace four minutes before the break, which McGee tipped over.
Scunthorpe were level eight minutes after the restart, though, as van Veen controlled Hopper's cross and ruthlessly dispatched his finish from eight yards.
Pompey were then indebted to McGee for making a superb reflex stop to keep out Morris' first-time effort from six yards.
The Blues went to a 4-1-4-1 formation after 72 minutes when Matty Kennedy and Adam May were introduced.
The visitors had the better of the second half, but Kal Naismith did threaten with a low drive which keeper Rory Watson held at the second attempt with 10 minutes left.
Pompey were the team under greater pressure through the game's finale, but there was no further action through five minutes of stoppage-time as it finished all square.
The draw means the Blues now sit seventh in the table, with Charlton's 1-0 win at Bury seeing them leapfrog Jackett's side to occupy the final League One play-off spot.