Brave Lupus Tokyo withstood a late fightback from reigning Japan Rugby League One champions Kubota Spears for a 24-20 victory on Sunday.
The Todd Blackadder-coached Brave Lupus saw out the last six minutes with 14 men after New Zealand flyhalf Richie Mo'unga was yellow carded for a dangerous tackle.
Former All Black Dane Coles opened the scoring for Kubota with his first try in Japanese club rugby before Brave Lupus grabbed the upper hand with two converted tries in a four-minute span at Todoroki Stadium in Kawasaki.
The first came when Frans Ludeke's team spilled the ball near their own line, with Brave Lupus scrumhalf Yuhei Sugiyama darting across and Mo'unga adding the extras for a 7-7 deadlock.
Brave Lupus went up 14-7 after finding space out wide with a quick attacking movement that culminated in Takuro Matsunaga going across.
Mo'unga and Kubota's Gerhard van den Heever traded penalties for a 17-10 halftime margin.
Van den Heever brought Kubota closer with a 54th-minute penalty, but the former Brave Blossom was caught out later in the half when an unfriendly bounce from Mo'unga's kick ahead prevented him gathering the ball and enabled Brave Lupus winger Masaki Hamada to touch down.
Spears quickly capitalized with a Liam Williams try after Mo'unga was marched for his hit on replacement back Halatoa Vailea, but they were denied further opportunities as Blackadder's men kept possession until the final hooter.
In other results, Springboks flanker Kwagga Smith dotted down twice for Shizuoka BlueRevs in their 62-13 win over Mie Honda Heat.
Fijians Inoke Burua and Viliame Takayawa each scored a brace of tries to spark Yokohama Eagles to a 40-35 comeback win against Sagamihara Dynaboars.