Hirai's clinical finish opened the scoring for the 2008 winners in the Group G encounter at Jalan Besar Stadium.

Erwan Gunawan headed SAF level after 30 minutes after home goalkeeper Shahril Jantan had denied Hirai, Korean international Cho Jae-jin and midfielder Takahiro Futagawa.

Gamba regained the lead just 10 seconds after the restart as Cho beat Shahril to a cross, leaving Hirai with a simple tap-in.

SAF again hit back as Swedish defender Niklas Sandberg headed home Ahmad Latiff's free-kick two minutes after the hour mark.

But Sota Nakazawa smashed home from point blank range for Gamba eight minutes later and, after Taisuke Akiyoshi was sent-off for the visitors, Hirai sealed his hat-trick with nine minutes remaining.

"During half-time, the players weren't nervous about the score being 1-1 because we knew we were controlling the game and playing well," said Gamba coach Akira Nishino.

"The team system could have been improved and we could have passed the ball around more effectively, but we weren't nervous at all.

"We haven't had a good season. Now we will aim to keep winning and climbing up the table in the Champions League."

With Suwon Samsung Bluewings beating Henan Jianye 2-0, the K-League side top the table with seven points two clear of Gamba.

At Zhengzhou Hanghai Stadium in Henan, Brazilian striker Jose Mota scored twice in the second half as unbeaten Suwon posted a second consecutive victory to crucially move five points clear of the third-placed Henan.

Mote broke the deadlock just a minute after the break as he headed home Juninho Vendrechovski's free-kick before sealing the three points a minute before the hour mark from Li Weifeng's left-footed cross.