Southampton moved back to the top of the Championship but were denied victory as David Norris rescued a point for Portsmouth to keep alive their slim hopes of survival.
Billy Sharp gave the Saints a first-half lead, poking home from close range but Chris Maguire levelled for the visitors with an outstanding strike from the angle of the box.
Sharp looked to have earned Saints victory with a controversial late strike, which was initially ruled out for offside, before Norris struck deep into injury time.
The result leaves Saints level with Reading on 79 points but ahead on goal difference and only four in front of third-placed West Ham.
For Pompey this was a point earned, with Michael Appleton's men still five points from safety with five games remaining.
And if they can keep Maguire and Scott Allan fit for the remainder of the campaign they could yet pull off an unlikely escape.
Nigel Adkins's Southampton remain favourites for promotion to the Premier League despite throwing away a lead twice.
The result also sets up an intriguing battle with Reading, who are only behind the Saints on goal difference, at St Mary's next Friday.
Saints began the day second in the Championship table, booted off top spot by Reading following their 2-0 victory over Leeds on Good Friday.
They welcomed back top-scorer Rickie Lambert from injury as they looked to avoid back-to-back defeats for only the second time this season.
Pompey had the first chance of the game, Kelvin Etuhu testing Saints keeper Kelvin Davis with a fierce drive from just inside the box.
At the other end, Saints looked lively with Sharp's movement in particular causing the Pompey defence problems.
The striker put the hosts in front, poking home Lambert's pass from close range after Pompey failed to clear their lines.
But Maguire was proving a useful outlet for Pompey and he soon got the visitors level with an outstanding strike, which found the top corner of the net from the angle of the box, although keeper Kelvin Davis will have been disappointed to be beaten at his near post.
Maguire should have put the hosts ahead just after the break but after finding space in the Saints box he sliced his side-foot wide of Davis's goal from 12 yards out.
The game soon opened up and Pompey had Ashdown to thank for pulling off two fine saves, first from Adam Lallana's volley and then from Jose Fonte's close-range header.
It was the closest Saints came for some time, with the game becoming scrappy as Pompey looked to take the sting out of the match.
In the dying stages Fonte had a great opportunity for Southampton but, unmarked, he volleyed over from close range. And with two minutes remaining Lallana opted to round the keeper rather than shoot but lost his feet and the ball ran harmlessly out of play.
It looked as if the game would end 1-1 but with time up Sharp prodded home from close range following Steve de Ridder's knock down from Lallana's corner.
The goal was initially ruled out for offside but after consulting his assistant, referee Neil Swarbrick awarded Sharp's effort.
However, there was more action to come and deep into injury time Norris kept his nerve to fire a left-footed volley from 20 yards past Davis to rescue an unlikely point.