Lukas Julis scored a hat-trick as Sparta Prague earned a 4-1 win against Celtic that all but ends their opponents' hopes of qualifying from Europa League Group H.
Pressure was already on Celtic boss Neil Lennon following a run of one win in five games and his team were again below par against a Sparta side who were without a number of regulars due to coronavirus.
Socceroos midfielder Tom Rogic played the full 90 minutes and was heavily involved in Celtic's only goal. Rogic also had two shots on target, 50 touches, 30 passes with 90% passing accuracy, five dribbles and one interception.
Julis opened his account in the 26th minute and doubled his tally on the brink of half-time with a curled finish, the goals coming either side of Mohamed Elyounoussi hitting the bar from a rare Celtic chance.
The hosts were given a route back into the game 25 minutes from time when substitute Leigh Griffiths made an instant impact, but Julis killed off the contest with his third and Ladislav Krejci added a late fourth to pile on the misery.
David Moberg Karlsson crashed an early shot against the woodwork and was denied with another effort by Scott Bain, who also kept out Julis with a brilliant reaction save.
However, Sparta's pressure told moments later as Julis lashed into the roof of the net after David Hancko's glanced header had come back off the frame of the goal.
Celtic orchestrated a chance 38 minutes in, Elyounoussi meeting Ryan Christie's pass and grazing the crossbar with his volley, but Sparta were soon two goals in front through Julis' fine finish into the bottom-right corner.
Michal Sacek was given space to fire a long-range shot against the post as Celtic came within inches of shipping a third, though that let-off acted as a turning point of sorts.
Florin Nita kept out Tom Rogic's shot but Griffiths was on hand to fire the rebound into an empty net against a Sparta side previously without a point to their name.
Just as Celtic had some momentum on their side, Karlsson got away from Shane Duffy and squared the ball for Julis to bundle in.
Substitute Krejci then got on the end of a Borek Dockal cross in the 90th minute to further the pain of Celtic, who have lost three home games in a row in the competition for the first time and sit bottom of their group.
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Lennon was keen to take positives out of his side's 2-2 draw away to in-form Lille last week, but he will struggle to find much to be pleased about after this chastening defeat.
Celtic looked leggy and were far too passive up until the point Griffiths temporarily revived their hopes, collectively managing four attempts on target to their opponents' eight.
The Scottish champions are bottom of Group H at the halfway stage, two points behind Sparta and five adrift of Milan, who suffered a surprise 3-0 home loss to new leaders Lille.
The Sparta striker had 35 touches of the ball - the fewest of any player to play the full 90 minutes - yet he managed to find the net from three of his five attempts.
Julis' opener was all about power as he drilled the ball high into the net, the second was more about precision and the third was a case of making himself free for a simple tap-in.
Leigh lifts Celtic
Lennon replaced Elyounoussi with Griffiths on the hour mark and the substitute striker needed just six minutes to do what Elyounoussi could not - find a way past Nita.
The departing Elyounoussi completed just 61 per cent of his 18 passes - the lowest passing accuracy rate of any Celtic starter - with the hosts' best spell coming when two players were fielded up top.
Celtic take on Motherwell on Sunday in their final Scottish Premiership outing before the international break, while Sparta travel to Viktoria Plzen the same day.