Naoto Tsuji had the hot hand early.
The Kawasaki Brave Thunders shooting guard sparked the hosts in the opening quarter and finished with a game-high 20 points in an 89-61 thrashing of the Tochigi Brex on Friday night.
Tsuji sank 4 of 5 3-point attempts in a 12-point first-quarter performance, helping the Brave Thunders (37-17) take a 27-20 advantage into the second stanza.
The Brex, who never led, trailed 46-34 at halftime.
Kawasaki's Nick Fazekas contributed 19 points, 13 rebounds, four assists and two blocks and Ryusei Shinoyama added 17 points and four assists. Josh Davis finished with 10 points and nine boards.
The hosts shot 56.9 percent from the floor, including 12 of 21 from beyond the arc.
The Brave Thunders won the rebounding duel by a 43-28 margin.
Jeff Gibbs paced Tochigi (30-24) with 15 points and snared seven rebounds. Yusuke Endo had 13 points and four assists and Ryan Rossiter added 10 points, nine rebounds and five assists along with a game-high five turnovers.
Orange Vikings 121, Wat's 110
In Matsuyama, Ehime Prefecture, Portland State alum Chehales Tapscott scored a season-high and second-division record 50 points and four teammates also reached double digits in points as the hosts defeated Aomori.
Tapscott, a 27-year-old power forward, entered the series opener averaging 21.5 points per game, No. 1 in the 18-team second division. He had a pair of 42-point games earlier this season and another with 40.
The 195-cm star converted 19 of 27 shots from the floor, including three 3-pointers, and sank 9 of 10 free throws in 32-plus minutes for the Orange Vikings (30-24).
Ehime's Yoshihiko Toshino added 17 points, Conor Clifford scored 15, Tatsuhiko Toshino finished with 13 and 11 assists and Nyika Williams had a 14-point effort.
For the Wat's (15-39), Kyle Barone, a University of Idaho product, scored 23 points and grabbed nine rebounds and Joseph Burton poured in 20 points. Daichi Shinoyama chipped in with 15 points and Lakeem Jackson had 14.