Playing on home soil, Guangzhou were unable to give their fans anything to cheer about as Seongnam took the lead in the 33rd minute courtesy of striker Hwang Ui-jo.
Even though they put up a better performance in the second half, Guangzhou were unable to find the back of the net and were subsequently handed their second consecutive loss.
The loss now put Guangzhou in third place on the Group F table with three points.
"Although we lost today, I still want to congratulate my players," Contra told the official tournament website. "They tried their best, and they wanted to win all the time. The match was very difficult as we conceded a goal even when we hadn't settled down at all and it caused lots of pressure. We had lots of shots, but their defense was very good.
"The early goal we conceded made it too difficult to us. If we could have scored one in the second half, we would have had another result.
"We still have three games to go, we have to win at least two of them and get a draw to qualify.
"R&F are new to the ACL. We also had a very early start to the season. We've tried our best to cope with the situation and I couldn't ask any more from my players. They have done everything they could."
Meanwhile, Seongnam coach Kim Hak-bum lauded his players for fighting through the humid conditions to pick up a much-needed win.
"The weather in Guangzhou is very humid and we overcame lots of difficulties to win three points thanks to my players," Kim said. "Hwang Ui-jo is in very good shape. He prepared enough for this game and I hope he can keep doing this and score more.
"We were very successful in defending as Guangzhou R&F tried very hard in attack, but they couldn't make it through.
"It is difficult to predict the next games. Our match schedule is very tight, and we can't easily foresee any situation, but if we win the following three games we will qualify.
"I know we are the underdogs, but as a team we are very confident to win any game against any team."