Both Kondo Racing drivers appeared in good shape after Saturday's dry qualifying session, as Yamashita and Sacha Fenestraz took second and third on the grid behind poleman Tomoki Nojiri.
But the wet conditions of the race proved a different story, as Fenestraz finished a distant fourth and Yamashita came away with no points after pitting on lap 11 of 31, despite tyre changes not being mandatory.
Yamashita was running well inside the points in fourth at the time of his trip to the pits, having initially held second before being passed by both Tadasuke Makino and eventual race winner Nobuharu Matsushita.
Speaking afterwards to Motorsport.com, Yamashita said that even if he had elected to stay out, his rear tyre struggles were so acute that he questioned if he would have scored points in an case.
"Within two or three laps of the start I could feel the rear tyres dropping and moving around, and the pace dropped a lot more than the cars around me," he explained. "After discussing with the team, finally the team decided we should pit, so I came in, but it didn’t work.
"Whether or not we pitted, the pace was so bad that I don’t know if we could have stayed inside the top 10. I don’t know the reason why the tyres dropped so much, but that was the problem.
"I was surprised at how much they were moving and how much they dropped. It felt like I couldn’t drive properly anymore. But anyway, the tyre felt ‘weird’. I think some other drivers had no issues, so we need to find the cause."
Inging driver Sho Tsuboi suffered an even worse drop-off than Yamashita, as he rose to fourth on the opening lap from sixth on the grid, only to plummet outside of the points positions before coming in to change tyres.
Tsuboi lost further time with an excruciatingly slow pitstop, finally finishing a lap down in 20th place.
"Suddenly I lost grip, and the pace dropped so much that I couldn’t do anything about it as a driver," Tsuboi recalled to Motorsport.com's Japanese edition.
"I was just trying to stay out of the way of the other drivers, but the tyres were so far gone that even that was difficult.
"In reality there was no slow puncture, but it felt like that when I was driving. I was so dangerously slow that I felt there was no choice but to pit."
Tsuboi's race engineer Yoshinari Suganuma admitted that Inging had chosen a setup suited to a wetter track, which proved the wrong call as conditions improved slightly as the race went on.
"I think the same thing happened to all the drivers that pitted in," Suganuma told Motorsport.com's Japanese edition. "It was to do with how our tyres were prepared. Whether you believed it would continue raining or would stop raining during the race changed a lot.
"We went too much on the ‘it will keep raining’ side, as since the time [that Tsuboi’s pace dropped] the amount of water on the track became less."
Fenestraz almost followed Yamashita into the pits
After a slow start from third on the grid, Fenestraz admitted that he wanted to copy Yamashita's strategy of pitting in as he likewise struggled with the tyres, only to be talked out of it by his team.
In the end, the Franco-Argentine driver managed to hang on to fourth place ahead of KCMG driver Kamui Kobayashi.
Fenestraz hypothesised to Motorsport.com that the conditions favoured those who had opted for lower tyre pressures, which would have made it harder to heat the tyres early on but would have helped prevent overheating late in the race.
"I think [Yamashita and I] tried to copy each other on the tyre pressures, but at the last moment I decided something different, which I think definitely helped me," he said. "But it was still difficult.
"After five laps I was already telling the team [over the radio], ‘I have no rear tyres, what do we do?’ but they told me everyone was having similar issues. My teammate boxed in front of me, and actually I wanted to box as well, but the team told me, ‘no, stay out, your pace is not that bad’.
"I think today the tyre pressures were the key, and I think we got it wrong. I think Matsushita went really low on the tyre pressures because he wasn’t that fast at the start, but then the last five laps he was unstoppable."
However, when asked by Motorsport.com about Fenestraz's theory, Yamashita didn't think this was the cause of his woes.
"Our pressures were a little bit higher, but really just by a tiny amount," he said. "I don’t think that was the reason for the bad pace."
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