Japanese duo Gamba Osaka and Kashiwa Reysol stole the show with each recording a crucial away win against Suwon Bluewings and FC Seoul, results which provided them with a clear advantage ahead of next week's return matches. Also impressing were Lekhwiya who edged past Al Sadd 2-1, while Iran's Persepolis and Naft Tehran both won by 1-0 at home at the expense of their respective Saudi opponents.

Elsewhere, Beijing Guoan came from behind to draw 1-1 at Jeonbuk Motors, but Guangzhou Evergrande lost 2-1 at Seongnam FC to Kim Doheon's stoppage-time penalty. Meanwhile, Al Ahli and Al Ain shared the spoils in a goalless draw leaving everything on the line in the second leg.

The match
Seongnam FC 2-1 Guangzhou Evergrande
This champions' clash had nearly every element of an epic encounter with two fine strikes, a red card and an injury-time penalty. Despite missing both star-striker Elkeson and Korea Republic international defender Kim Young-Gwon due to injury, visiting Guangzhou proved the brighter side in the opening stages. Hosts Seongnam, however, took the lead against the run of play midway through the first half when Jorginho slotted past goalkeeper Zeng Cheng. Fabio Cannavaro's visitors, for their part, maintained their dominant play with Huang Bowen leveling with a 30-yarder three minutes from the break.

The match was, though, tipped in Seongnam's favour on 65 minutes when Guangzhou were reduced to ten men following Li Xuepeng's dismissal. Kim Hak-Bum's hosts had the better of the remaining period with both captain Kim Do-Heon and Jorginho looking dangerous as the Chinese remained on the back foot. Just as the game looked destined for a draw, Guangzhou conceded a spot-kick during a goal-mouth scramble from which Kim scored the match-winner.

The surprise
Suwon Bluewings 2-3 Kashiwa Reysol
Three-time AFC Champions League campaigners Kashiwa Reysol may not be among the most accomplished sides from Japan, with a last-four finish in 2013 their best result in the Asian stage. However, Tatsuma Yoshida's visitors boast arguably the best record against K League rivals among all Japanese teams, having won six and drawn one of their previous nine meetings. Despite languishing in 13th place on the domestic table, Kashiwa showed contrasting form on the continental scene by defeating Suwon 3-2, a side whom they trashed 6-2 in a group game two years ago.

Aiming to avenge that disastrous loss, Suwon got off to the start they had expected when Yeom Ki-Hun put the hosts ahead inside a minute. But Kashiwa responded soon as Akimi Barada drew them level on 12 minutes. And it didn't take them too long to take the lead when Leandro converted a spot-kick. The Brazilian forward struck again in the second half to make it 3-1. Jong Tae-Se was then on target to put the game on a knife-edge but the Japanese would hang on to again prove Suwon's nemesis.

The other attractions
Also suffering defeat were FC Seoul, who fell to Gamba Osaka 3-1 at home putting their campaign into real jeopardy. Following a goalless first half, the Japanese champions came to life when goals from Takashi Usami and Koki Yonekura in the space of 11 minutes gave them a 2-0 lead. Usami then made it 3-0 with his second four minutes from time, though Yun Ju-Tae's stoppage-time goal gave them a foothold in the tie.

Jeonbuk Motors's hopes were also dealt a blow as they were held to a 1-1 draw by Beijing Guoan at home. The 2006 winners made a dream start when Kim Kee-Hee headed the home side in front. The Chinese, unsurprisingly, pressed forward thereafter and their efforts paid the dividends with four minutes remaining when Pablo Batalla scored the equaliser from the spot for a precious away point.

In the continent's west, a stoppage-time own-goal by Digao saw last year's runners-up Al Hilal suffer a 1-0 loss at Persepolis despite dominating the majority of the game. The Iranian hosts were indebted to goalkeeper Sousha Makani who repeatedly frustrated the Saudi attackers. Lekhwiya downed Al Sadd 2-1 in an all-Qatari affair leaving Michael Laudrup's visitors in the driving seat for the return. All three goals were scored in the first half, with Sebastian Soria opening the scoring early before Youssef Msakni doubled the lead. Hassan Al Haydos pulled one back immediately but the equaliser didn't come as the hosts suffered their first defeat on home soil in the campaign.

Elsewhere, the two teams named Al Ahli both disappointed in their respective matches. Saudi’s Ahli lost 1-0 at Naft Tehran with the tournament debutants continuing their impressive run. Iran international Gholamreza Rezaei scored the only goal in the 34th minute which provides his side with a slight edge heading into next Wednesday's return in Jeddah. Al Ahli from UAE, meanwhile, could only manage a goalless tie against inaugural winners Al Ain at home, a result which leaves the second leg wide open.

The player
While Gamba's prospects of a second Asian title are bolstered by an emphatic victory at FC Seoul, the 2008 champions were left to thank Takashi Usami as the in-form striker nabbed a brace. Having racked up 23 goals in all competitions as his side won the domestic treble last season, the 23-year-old former Bayern Munich attacker has maintained that prolific form this campaign and currently tops the J-League scoring chart with ten goals.

The stat
- Three goals were scored deep into stoppage-time with two of which proving match-winners.

What they said
"The players are angry and annoyed because it was a tough match and we had a red card and a penalty. But I trust they will use this anger in the return when we play at home," Guangzhou Evergrande coach Fabio Cannavaro

Round of 16 first-leg results:
May 19 & 20
Jeonbuk Motors 1-1 Beijing Guoan
Suwon Bluewings 2-3 Kashiwa Reysol
Persepolis 1-0 Al Hilal
Al Sadd 1-2 Lekhwiya
Seongnam FC 2-1 Guangzhou Evergrande
FC Seoul 1-3 Gamba Osaka
Naft Tehran 1-0 Al Ahli
Al Ahli 0-0 Al Ain