Yamamoto and Hirakawa were battling over third place on lap 27 of 37 when the pair made contact at the Turn 11 right-hander, the result of Yamamoto making a late lunge to the inside.
Impul man Hirakawa was tipped into a half-spin but otherwise was able to continue, but Yamamoto was only able to get far as the start/finish straight before his damaged front-left suspension gave way, forcing him out of the race.
At the time of the incident, Hirakawa was on his out lap after a slow pitstop that dropped him behind Toshiki Oyu and into the clutches of Yamamoto, who had pitted on lap 24 and had the advantage of warm tyres.
Yamamoto was deemed to be at fault for the clash by the stewards, but the three-time champion escaped with only a warning.
"I was trying to take advantage of my tyre advantage, and I heard on the radio about Hirakawa’s slow pitstop," said Yamamoto. "I felt I had no choice but to fight and I decided to try and dive up the inside.
"Of course, I understand that if you are too far back, you can’t force a move. But I could see my rival, who was still warming up his tyres, braked early and I thought it was doable from that distance.
"As soon as I braked, I was forced towards the inside and there was nowhere for me to go except for the grass. I didn’t lock the brakes and I thought I could pull it off, so it’s a shame.
"But when you’re behind your rival, the inside is open and you don’t go for a move, you’ll be looked down upon, so I thought there was no choice but to try."
For his part, Hirakawa said he was taken by surprise at Yamamoto's passing attempt, saying he felt already up to speed by the time the pair reached the traditional main passing point at Motegi.
"I was waiting for my tyres to warm up," recalled Hirakawa. "But by the time I reached the S-curve [Turns 7-8], the tyres were already feeling relatively good and I used the Overtake System and tried to block Yamamoto.
"By the time we reached Turn 11, the tyres were up to temperature, and I thought there’s no way he would come at me from that distance. I didn’t think he would try and force the issue on the inside. I was surprised.
"After the collision, I was able to calmly deal with the situation and minimise the time loss. I just didn’t think he would come from that far back."
Yamamoto added that he felt he could have finished second had his move on Hirakawa succeeded, as Toshiki Oyu struggled for pace - ultimately slipping behind Hirakawa in the closing stages and finishing third.
"If I had been able to pass Hirakawa there, I think I could have passed Oyu as well, so it could have been an even better result," lamented the 35-year-old, who has now gone a year without standing on the podium.
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