Celtic's bid for Champions League football is in grave danger following a 1-1 draw against AEK Athens in the first leg of their third qualifier at Parkhead.
Brendan Rodgers' side came out of the traps with pace and purpose and midfielder Callum McGregor drove in the opening goal in the 17th minute, but that good work was undone two minutes before the break when Victor Klonaridis fired in a breakaway equaliser.
The visitors were reduced to 10 men in the 57th minute when Konstantinos Galanopoulos picked up a second yellow card from Italian referee Luca Banti for tripping James Forrest as he raced towards the AEK penalty area.
Celtic dominated possession but could not get the winner against a well-organised Greek side and have a big task ahead of them in the return game in Athens next Tuesday if they are to reach the play-off against either Malmo of Sweden or Hungarian side MOL Vidi.
Amid a frenetic start to another big European night in the east end of Glasgow, left-back Kieran Tierney, a constant source of danger down the flank, stung the hands of AEK goalkeeper Vassilis Barkas with an angled drive from inside the box, with defender Vasilios Lampropoulos completing the clearance.
Moments later, McGregor's drive from a similar distance was parried over by Barkas for a corner, which was defended with some desperation.
It was the two energetic and enterprising Hoops players who combined for the opening goal, Tierney again working himself into a good position inside the box and his pass was pounced on by McGregor, who swivelled and fired the ball low into the corner of the net from 10 yards.
AEK, under new boss Marinos Ouzounidis, who in his first competitive game in charge brought in Swedish defender Niklas Hult for Lucas Boye as a late change, wobbled.
Forrest fired wide of the far post in the 39th minute, seconds before Hoops keeper Craig Gordon tipped a powerful 25-yard drive from Helder Lopes over the crossbar for a corner which eventually came to nothing.
However, Lopes had more luck in the 43rd minute when he raced down the wing to collect a long pass from Andre Simoes and cut the ball back for the unmarked Klonaridis who lashed it past Gordon from 12 yards.
While some sporadic warning signs had been present, the goal stunned most of Celtic Park and changed the complexion of the tie.
AEK came out for the second half with a spring in their step but Celtic soon had them on the back foot again, the excellent Tierney leading the charge once more.
Another twist came when Galanopoulos, booked moments earlier for a foul on Tom Rogic, was sent packing for a cynical trip on Forrest and Parkhead was energised again.
Leigh Griffiths replaced Rogic to partner French striker Odsonne Edouard and Celtic swarmed around the Greek side's penalty area, Olivier Ntcham again coming close with a long-range effort.
AEK, in their first competitive game of the season, sat deep, happy to keep the scoreline intact.
The home side threw themselves forward during the final stages and five added minutes but the tiring visitors defended with discipline and diligence and will fancy their chances of progressing in front of their own supporters next week.