Kimura has been recruited to join Tadasuke Makino aboard the Team Kunimitsu Honda NSX-GT at both Autopolis and next month's Motegi season finale in the wake of Yamamoto's major crash last time out at Sugo.
After racing for the now-defunct ARTA GT300 operation last season, Kimura has spent 2023 on the sidelines as a Honda reserve driver in the top class while focusing solely on his main Super Formula Lights campaign.
The Russian-Japanese was registered as a third driver for ARTA at Fuji in August when Toshiki Oyu's participation was in doubt due to his prior collarbone injury, but in the event Oyu was fit to drive, although Kimura turned a handful of laps during the full-course yellow practice session.
Kimura was once again on standby at the next round at Suzuka, this time with Team Kunimitsu when Makino risked missing the race due to his Super Formula crash the previous week at Motegi.
Now, following Yamamoto's accident at Sugo that has left the Honda talisman driver nursing significant neck injuries, Kimura gets his chance in the limelight racing for the brand's de facto number one team.
Day one in the office was predictably tough however, as Kimura was eighth and slowest in Q2 after Makino set the fourth-fastest time in the opening segment.
"In free practice I was able to drive a lot and get used to the used tyre, and make good constant laptimes," Kimura told Motorsport.com.
"But in qualifying, I pushed hard but it was too much, I locked up and I destroyed the tyres. That was just my mistake.
"It was really tough, even more than I imagined. In practice I did a 1m34.1s, but pole was a 1m31.1s, so three seconds faster than anything I had experienced. I couldn't imagine how to go that fast. That was really tough for me."
On the plus side, Kimura kept his heels clean around the tricky Autopolis circuit, which is more than can be said for Oyu, who gave his ARTA mechanics a hefty repair job after a shunt in free practice.
Asked what targets the team has set for him, Kimura emphasised the importance of getting through the day without major incident, even while Makino remains in outside title contention, 20 points behind the championship-leading NDDP Racing Nissan crew.
"Of course the team wants to finish on the podium and I know the car has the performance to do that," he said. "But the team knows I am still learning.
"The team has asked me not to crash, because if I crash, I lose the chance to do more laps and gain experience. I am really focussed on that, just learn step-by-step and not push too much, even if I did that in qualifying a little bit!"
Kimura had a slight head start for his GT500 debut by virtue of the fact he spent the Suzuka weekend with Team Kunimitsu, even if he didn't end up driving with Makino only sitting out the Saturday before returning to action for race day.
"Their approach is different to ARTA, so I was really good that I was able to experience working with Team Kunimitsu already," he added. "Makino has also been giving me a lot of advice which is a big help for me, like Yamamoto-san has done for him."
For Kimura, the timing of his chance with Team Kunimitsu is significant, as it puts him in the shop window just as Honda mulls its GT500 line-ups for 2024.
In Super Formula Lights, he trails Toyota youngster Hibiki Taira by 10 points with just next month's Motegi finale to run. But he feels getting two races under his belt in GT500 machinery in that time could give him the edge he needs to overhaul his rival.
"I need to make a good result in Super Formula Lights," he replied when asked about the significance of his performances at Team Kunimitsu for his future. "But even if I do good results here, if Lights is bad, there could be no chance. I’m not sure what Honda will decide.
"But the good thing is that the last Lights race [at Okayama] was in mid-September and the next race is in November [the week after the Motegi] SUPER GT finale. I wasn’t supposed to drive anything for two months, but now I have the chance to drive the GT500.
"Even if it’s really difficult, I can apply some techniques to Lights. So I can be confident about winning the championship too."
Kimura was a driver under pressure heading into the current season. After losing out to Kakunoshin Ota in the battle to be Honda's top junior driver in Super Formula Lights in 2022, he faced a similar challenge from another member of the marque's stable, Shun Koide.
The momentum has been firmly on Koide's side in recent rounds with four wins from the last five races, but Kimura's unexpected call-up to the GT500 ranks offers him a chance to turn the tide and stake his claim to be the next Honda junior to get the call up to the big leagues.
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