Josep Gombau's men were forced to share the spoils in Adelaide despite playing the final 20 minutes with an extra man
After Ranko Despotovic got the visitors on the board early, the Reds, already without Argentine pair Marcelo Carrusca (groin injury) and Jeronimo Neumann (suspended), lost in-form striker Bruce Djite (groin) and gun playmaker Sergio Cirio (hamstring) in the first half-hour.
But Fabio Ferreira, returning from an adductor injury, conjured the equaliser for the hosts, before Jake Barker-Daish converted a penalty in the 72nd minute.
Down 2-1 and with Nick Carle expelled from the contest in the 68th minute, 10-man Sydney looked dead before substitute Corey Gameiro nodded home the visitors' second goal in the 82nd minute to draw level.
Sydney's midfield controlled play early with some slick passing and it didn't take long for them to capitalise.
Eight minutes in, Richard Garcia, out wide on the left, whipped a low cross which Despotovic gobbled up and punched home, with United goalkeeper Eugene Galekovic out of position, giving the Sky Blues a perfect start.
Despotovic had the chance to net his brace when he was put through by Sky Blues skipper Alessandro Del Piero in the 16th minute but the Sydney striker was ruled offside.
Adelaide were under the pump and they received another blow when Djite, the Reds' sole goalscorer in last week's win over Newcastle, hobbled off the park and straight up the race in the 20th minute, replaced by jet-heeled youngster Awer Mabil.
Despite the setback, the Reds looked determined to equalise and did so in the 24th minute when Ferreira got on the scoresheet in his comeback from injury.
Steven Lustica's searing long cross found Ferreira at the right post and the Portuguese winger leapt high for a fine scissor volley, which brushed the post and found the back of the net past Sydney gloveman Vedran Janjetovic.
Just as Adelaide began to dominate proceedings, they were dealt another significant blow when star recruit Cirio limped off with a right hamstring injury at the half-hour mark.
The Reds' hearts were in their mouths in the 37th minute when Despotovic went down in the box following a sliding Galekovic stop, but there was no whistle.
From the ensuing corner, Nikola Petkovic nonchalantly turned on his left and fired a long missile which beat the sprawling Galekovic but collected the crossbar.
United were relieved at the interval and looked switched on after resumption, with Mabil repeatedly looking dangerous streaming down his left flank, engaging in a fine duel with experienced Sydney right back Pedj Bojic.
But the Sky Blues weathered the storm and mustered two good strikes in the 56th minute by Despotovic and Garcia, both superbly saved by Galekovic.
At the other end, Ferreira burst away on a sizzling counter and was fouled by Bojic, who copped a yellow for his challenge. Adelaide conjured three half-chances from the ensuing free kick but were unable to get adequate traction on any of their shots in the danger zone.
The see-sawing contest turned dramatically in the 68th minute when Carle was shown his second yellow card for hanging on to Isaias, the Sky Blues down to 10 men.
The undermanned visitors were in further trouble when Matt Jurman was called for a foul from behind on debutant Anthony Costa in the area.
Barker-Daish duly stepping up to the spot to take the penalty and drove it into the top right-hand corner to give Adelaide the lead.
But the Sky Blues showed tremendous resolve and had to go for broke, which paid off when dead-ball specialist Petkovic delivered a stinging free kick which was headed home by Gameiro to make it 2-2 with eight minutes remaining.
Gameiro had two further chances to steal a last-ditch victory but Galekovic was up to the task on both occasions and a draw was a just result for both camps who threw everything at each other in the face of doubled-edged adversity.
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