The Italian international, who inspired a stunning fightback from two goals down against Melbourne Victory in his tournament debut, buried a superbly struck free-kick to cancel out Jin Hanato's opener. New recruit Diamanti, 30, playing under ex-Italy manager Marcello Lippi, now has three goals in two appearances in the Asian competition, doing no harm to his chances of selection for the FIFA World Cup™ later this year.
However, the defending champions dropped their first points and they currently lie second in Group G on goal difference. Jeonbuk Hyundai Motors are narrowly ahead following their 2-2 draw with Victory.
Elsewhere, 2011 champions Al Sadd of Qatar were also held by the UAE's Al Ahli in their Group D match Dubai, with Ecuadorian Grafite clinching a 68th minute equaliser off an Ismaeel al-Hamadi assist. Earlier Nadir Belhadj had put Al Sadd ahead in the 24th minute off a pass from Rodrigo Tabata
In another Group D match, Iran's Foolad Sepahan edged Saudi Arabia's Al Hilal 3-2, thanks to a stoppage time strike by Albanian Xhevahir Sukaj. Sukaj's compatriot Ervin Bulku had put the Iranian side ahead in the 17th minute, but Al Hilal scored twice in five minutes through Segundo Castillo and Thiago Neves to entertain hopes of a win. However, after Mehdi Sharifi scored the equaliser in the 72nd minute, Sukaj put paid to Al Hilal's hopes with his late winner.
In Group C Qatar's Lekhwiya also dropped points, drawing with Iranian side Tractorsazi in Doha ending in a goalless draw, while Saudi Arabia's Al Itthad beat Al Ain 2-1 with Mukhtar Fallatah scoring both goals for the winners in Mecca.
Ulsan Hyundai extended their tournament-record winning streak to 11 matches, and Australia's Western Sydney Wanderers celebrated their first ever victory in the competition. Ulsan's run dates back to 2012, when they won the competition before failing to qualify for 2013, and Japan's Kawasaki Frontale came close to halting it when the game was goalless deep into the second half.
But with just six minutes left, substitute Yoo Jun-Soo ghosted into the box to meet a cross from the right with a well-directed header which sailed into the back of the net. Kim Shinwook ensured it finished 2-0 when he got Ulsan's second in stoppage time following a poor clearance by Kawasaki goalkeeper Yohei Nishibe.
In Yokohama, the hosts went ahead after 21 minutes when Hanato punished Guangzhou's failure to deal with a Shunsuke Nakamura free kick by volleying into the net from close range.But midfielder Diamanti, the star of Guangzhou's 4-2 comeback win against Victory, put the visitors level on 38 minutes with a dipping, left-footed free-kick which went in off the underside of the crossbar.
In Melbourne, a first-half header from Nick Ansell gave Victory a barely deserved half-time lead but they went behind in three second-half minutes after a brace from Jeonbuk skipper Lee Donggook. But with eight minutes left, New Zealander Kosta Barbarouses fired between the legs of Jeonbuk goalkeeper Choi Eun-Sung to snatch a point against the 2006 champions.