The Argentine superstar was not expected to play but eventually proved decisive after a dramatic 90th-minute spot kick
The Argentine was a shock starter after Guangzhou management ignored a club-imposed nine-game ban for ill-discipline in their last ACL encounter, and ironically it was Conca who stepped up for the dramatic late spot kick which earned Guangzhou top spot in Group H and a home Round of 16 clash.
Earlier, after a goalless first half, Gao Lin had raced down the left flank to put the Chinese side ahead on 49 minutes, sliding a low shot past Buriram keeper Sivaruck Tedsungnoen after Conca's clipped pass on the halfway mark.
However, the hosts, who had been eliminated following Kashiwa Reysol's win at Jeonbuk Motors earlier in the evening, levelled seven minutes later when Zheng Zhi's poor headed clearance fell to Suriya Domtaisong and he volleyed home from inside the penalty area.
Guangzhou, whose owner had said they'd reach the quarter-finals at the very least prior to this season's ACL, desperately threw Brazilian striker Muriqui on after 65 minutes despite a thigh problem which kept him out of the starting line-up, with their survival on the line.
It was end to end with Cleo tapping in from close range but being ruled offside, while Buriram substitute Sarif Sainui sliced wide on 80 minutes.
Then dramatically in the last minute of normal time, Surat Sukha pushed Cleo in the box earning the visitors a penalty which Conca fired powerfully home, despite Tedsungnoen getting a touch on it, for the decisive goal.