Diao struck in the dying seconds to earn ten-man Stoke an unlikely and cruel victory after Robert Huth's header had cancelled out Frederic Piquionne's first-half opener.
Portsmouth do not play at home again until after their reappearance at the High Court to answer the winding-up order issued by HM Revenue and Customs on March 1.
The home side began the day being told their plea to sell players outside the transfer window had been thrown out by the Premier League, leaving owner Balram Chainrai searching for another way to settle the unpaid £12million tax bill which is threatening their existence.
Club sources claim an unnamed South African consortium are close to completing a takeover to rescue the 2008 FA Cup winners.
The enormity of the task facing manager Avram Grant to retain Premier League status was demonstrated prior to kick-off by the award of the supporters' player of the month for January.
Step forward Asmir Begovic, sold to Stoke on transfer deadline day to help pay his former team-mates' wages and a sheepish recipient of the accolade on his return to the south coast, albeit it as a substitute.
Pompey fans do not have to search too hard for further reminders of their glorious recent past, with giant portraits of England duo Glen Johnson and Jermain Defoe adorning the stairwells of their dilapidated stadium.
But what Grant's hastily assembled team of foreign loan signings and players discarded by their Premier League rivals lack in quality compared to their illustrious predecessors, they make up for in heart and endeavour.
Grant could have been forgiven for thinking fate was conspiring against him when Piquionne's neatly taken 20th-minute effort was wrongly ruled out for offside.
Piquionne did not have to wait long to gain his retribution, reacting quicker than the Stoke defence when Thomas Sorensen could only parry Quincy Owusu-Abeyie's shot to side-foot home from an acute angle.
It took until the 40th minute for the visitors to register their first shot on target as David James, himself a January target for Tony Pulis, beat away Tuncay's close-range shot.
Tuncay should have equalised on the stroke of half-time when he put a free header over the crossbar from Glenn Whelan's cross.
Centre-back Huth showed Tuncay how it should be done five minutes after the interval, evading the marking of opposite number Marc Wilson to nod in Whelan's corner.
Stoke were forced to play out the final 17 minutes with ten men when defender Andy Wilkinson was sent off after collecting two yellow cards in the space of nine minutes for fouls on the lively Aruna Dindane.
But it was Pulis' side that cashed in as Portsmouth poured forward in search of a winner to prolong their faint Premier League survival hopes, with Diao sliding in at the back post to convert Ricardo Fuller's centre.