First-stage champions Urawa Reds were forced to work for a 2-1 win against promoted Matsumoto Yamaga on Saturday to preserve their unbeaten record in this season's J-League.

Goals from Yuki Muto and Shinzo Koroki secured the three points for last year's Japanese runners-up in their second-stage opener as Yamaga played with a back six in a bid to frustrate the visitors.

"It was a bit of a tap-in goal but it went in and that's what counts," Koroki told reporters. "But the fact we ground out the result shows the mental strength we have this season."

Meanwhile, 2014 champions and domestic treble winners Gamba Osaka needed a last-minute winner from Japan striker Takashi Usami to beat visiting Ventforet Kofu 2-1.

Brazilian pair Elsinho and Renato were on target as Kawasaki Frontale opened the second half of the season with a 2-0 home win over FC Tokyo, who lacked a cutting edge in attack after they sold Japan international Yoshinori Muto to Germany's Mainz.

The result was a tonic for Kawasaki, who were embarrassed 6-0 by Borussia Dortmund in a preseason friendly in midweek.

"We got a lesson from Dortmund," said Frontale coach Yahiro Kazama. "It was precisely the type of training we needed."

Twice champions Sanfrecce Hiroshima won a shootout away to Vegalta Sendai, prevailing 4-3 in northern Japan, while promoted Shonan Bellmare won 2-1 at home to Nagoya Grampus.

Vissel Kobe were the day's biggest winners, smashing five unanswered goals past Shimizu S-Pulse, who finished the first stage bottom of the first division and look set for a relegation dogfight.