Nojiri wound up all the way down in 16th place in Friday's 90-minute session, which marks the only running prior to Saturday morning's qualifying session for the penultimate round of the season.
The Mugen driver needs to score 15 points across the two races this weekend at Suzuka to complete a first successful title defence in Super Formula since Tsugio Matsuda in 2008, as he holds a 32-point margin over Sacha Fenestraz.
However, after ending up more than a second off pacesetter Toshiki Oyu in practice despite suffering no obvious mishaps, Nojiri admitted that the situation looks as bad as the timesheets suggest.
"As the results show, things are really bad," Nojiri told reporters. "From the start of the session, I thought, ‘damn, this is bad’. I ran properly on new tyres, and this was the result, so now we’re wondering what to do.
"The car’s level is pretty bad, so I don’t think it’s realistic to target being at the top, but I’m hoping to get to a level where we can compete at least to some extent. We’re not getting anywhere with the current set-up, so we’re making some changes. We need to find the issue, right now we have no idea.
"I’m pretty anxious. I made the kind of mistake I don’t usually make in practice. As I was driving, I thought, ‘what does ‘driving normally’ even mean?’
"I don’t want to be confident. We’ve been able to turn things around [after bad practice sessions] this year, but it could have just been a coincidence."
Explaining Nojiri's issues in more detail, race engineer Toshihiro Ichise admitted that attempts to rectify the weaknesses of the #1 car's set-up on Super Formula's previous visit to Suzuka in April had backfired.
"The problem is the balance is not consistent," Ichise told reporters. "We had oversteer on entry, understeer in the middle of the corner and oversteer on exit. You can't attack with confidence like this.
"The idea was to take what worked well in Round 3 and fix the bad points, so we just changed a little to try and make Sector 3 faster. After that, we adjusted it until it was almost back to how it was before, and it didn't work."
Title rivals taking no comfort from Nojiri struggles
Nojiri's two rivals for the championship, Fenestraz and Ryo Hirakawa, ended practice third and fifth respectively.
However, both drivers told Motorsport.com that seeing Nojiri suffering so much has made no difference to their outlook of snatching away the title, both predicting the reigning champion will rebound on Saturday.
Kondo Racing's Fenestraz said of Nojiri: "I would be surprised if he qualifies that low tomorrow. I won’t be surprised if he is back to where he is usually. I know he is quick, and Honda is likely to be quick this weekend in the colder conditions."
The Kondo Racing driver added he was "not happy at all" with his best lap despite ending up third behind Oyu and Sho Tsuboi.
"My sector one was terrible," admitted Fenestraz. "It was very up and down because there was a lot of headwind going into Turn 1, so sector one was really hard to be consistent. I had a lot of instability, the car didn’t feel good at all.
"We’ve been changing a lot of things around to find a good balance but nothing worked. Let’s see tomorrow, it’s an important day if we want to stay in the fight for the championship, so we’ll do our best."
For his part, Impul man Hirakawa, who is 34 points down on Nojiri with a maximum of 46 available this weekend, is hoping to turn around his recent poor qualifying form to keep his title challenge alive.
"Nojiri might have been struggling but I expect he’ll be ok for qualifying tomorrow, so I can’t be too optimistic," he said. "Maybe now there is a bit more chance that he can be slow [in qualifying], but he can also be fast like usual. So I just have to do my job, nothing really changed.
"If I am starting on the first or second row, I can be ok for the race, so the target is to qualify at least P4. As always, I'm confident I can be good in the race, so it’s just qualifying that’s my main focus."
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