A late goal from Sporting Lisbon substitute Luis Cartez denied Celtic a point in the opening match of their Next Gen campaign.
Celtic had taken the lead in the 23rd minute through Denny Johnstone but on the stroke of half time the visitors levelled through Iuri Medeiros.
Sporting had dominated the first half but it was the home side who looked the most likely to score after the break until Cortez struck in the 78th minute.
With away wins over Celtic and Aston Villa Sporting look like favourites to top the group with Celtic next in action at Villa Park on September 19.
Sporting dominated the early possession but found the Celtic defense disciplined and sticking doggedly to their task.
Goalkeeper Leo Fasan pulled off three fantastic saves during the first half but was out of luck with both goals as the young Celts paid a heavy price for being unable to convert their second half superiority into goals.
In the 15th minute Fasan did well to push a dipping 25 yard free kick from Farley Rosa over the bar before punching away a dangerous inswinging corner.
Midway through the first half Celtic took the lead. Callum McGregor broke down the left with Johnstone controlling and turning onto McGregor’s cut back to shoot high into the net for his third goal of the season.
Going behind didn’t effect Sporting’s style of play with Fasan called into action in the 28th minute to claw away a netbound shot from Carlos Mane.
While Sporting enjoyed the lions share of possession Celtic’s counter-attacking carried real menace and purpose but on the stroke of half time the visitors levelled when the hoops defence was opened up and Medeiros finally beat Fasan from 15 yards out.
Celtic started the second half on the front foot with the Sporting defence struggling to pick up McGregor as he pushed forward to support Johnstone.
Twardzik was pushing forward at every opportunity down the left and was brutally chopped down by Ruben Semedo in the 58th minute earning the Sporting defender a yellow card but Joe Chalmers failed to find the target with the free kick.
Celtic were dominating for long spells but 12 minutes from time Sporting sub Luis Cortez struck a low 25 yard drive that flew past Fasan into the net.
In the final minute of normal time Twardzik flashed a shot across the face of the goal with the night ending in disappointment for the home side as Sporting ran the clock down to claim the points..
CELTIC: Fasan; Fisher, Findlay, Fraser, Chalmers: Herron, Kidd, McGregor, Irvine, P Twardzik: Johnstone
SUBS: Lindsay, Daniels, Miller, McNally, Henderson, Kirwin, Waters
