Celtic lost the first match of its Australian tour after gifting Central Coast Mariners a late goal.
A poor clearance from substitute Joe Ledley allowed Troy Hearfield to fire home in the 86th minute.
Celtic went closest in injury-time when Gary Hooper saw a shot cleared off the line before a penalty claim was denied.
Neil Lennon handed Adam Matthews and Kelvin Wilson their first taste of action for the club as his team failed to find a cutting edge despite superior possession.
Former Cardiff full-back Matthews put in an accomplished display in the first half and unleashed a powerful long throw on several occasions, while Wilson made some decent interventions.
Celtic fans appeared to outnumber the Mariners support in a crowd of around 25,000 in the neutral ANZ Stadium in Sydney as their team began their pre-season campaign six weeks to the day after their Scottish Cup final triumph over Motherwell.
Lennon left out Thomas Rogne and Charlie Mulgrew as a precaution after they reported stiff muscles, while Kris Commons remained on the bench for the same reason.
Daniel Majstorovic, Georgios Samaras, Cha Du-ri and Ki Sung-yueng are still to join up with their team-mates following international commitments, while Emilio Izaguirre is recovering from a hamstring injury.
Matthews started at right-back, forcing Mark Wilson to switch to the left, while former Nottingham Forest centre-back Wilson lined up alongside Glenn Loovens, with Lukasz Zaluska in goal.
Richie Towell started in right midfield with James Forrest on the left flank outside Scott Brown and Beram Kayal. Shaun Maloney and Daryl Murphy started up front.
Injury deprived Lennon of the chance to see 18-year-old Mariners midfielder Mustafa Amini, who has been recommended to the Hoops boss.
The Australian side featured Michael McGlinchey, who began his career at Celtic Park, and former Falkirk striker Daniel McBreen.
Celtic carved out the first chance when Mark Wilson intercepted a pass and slipped the ball through for Murphy, but the striker's touch allowed the goalkeeper to smother.
Murphy was soon handed another chance after a clever pass over the top from Kayal allowed Forrest to cross, but the former Sunderland striker glanced the ball wide.
Celtic moved the ball about quickly without breaking last season's A-League Grand Final runners-up down, and Mariners came into the game towards the break. Kelvin Wilson did well to block a strike from Mariners newcomer Adriano Pellegrino, who was linked with Celtic last year before having a trial with Motherwell.
Lennon brought on Paddy McCourt, Anthony Stokes and Hooper for the second half. Matthews, Kayal and Murphy came off with Towell moving to right-back and Maloney dropping into midfield.
Celtic stepped up a gear immediately after the break without carving out a real chance, but Mariners settled down and Celtic soon had a big escape when Pellegrino was off target with a free header from a good position.
Ledley then replaced Maloney before Dominic Cervi came on for Zaluska.
Forrest fired a yard over from just outside the box, but Hearfield then found the top corner after Ledley failed to deal with a low cross from the left.
Ledley almost made amends when he flicked the ball into Hooper's path but the striker's finish was cleared off the line.
Stokes played the ball back in before claiming for handball and caught the ball in expectation of a penalty.
But none was forthcoming, just a free-kick for the Mariners, who held out for the victory.
