Neuville started the morning sitting third but a stunning effort on stage nine, held in tricky frosty conditions, brought the Belgian into the victory fight.
He took 18.8s out of Toyota’s Sebastien Ogier to climb to second and was 9.6s quicker than rally leader Elfyn Evans.
Neuville then assumed the rally lead on stage 10, before extending that advantage to 5.1s come the end of the morning loop of stages.
“It [stage nine] felt good but it didn’t feel that good,” Neuville told Autosport.
“I didn’t expect to have such a big gap.
”I expected the others to do well as well, but I had good information this time compared to yesterday and that makes a difference clearly.
“We saw Ogier take 17 seconds out of me and 11 seconds in three kilometres because my pacenotes were too conservative. I guess that was difference this morning.”
Evans’ slide back to second was assisted by a hybrid failure in stage 10, although the Welshman confirmed the power returned for the following test.
“Of course, the first stage this morning was pretty difficult to judge, and it looks like Thierry did a good job,” Evans told Autosport.
“We were not so bad but then we struggled obviously in the middle one which was costly for us.
“Of course, we are pushing anyway. I’m not planning to change things massively [for the afternoon] but I want to have a clean run.”
Now sitting third, 7.7s adrift of Neuville, Ogier believes he will need to drive perfectly to secure a 10th Monte Carlo win.
“It is still possible there is only eight seconds, but for sure from now there is no time to lose anything it is time to do everything perfectly and push to the end,” said Ogier.
“It is shame to lose a lot in one stage this morning, but other than that every stage is pretty intense. This [battle] is nice to follow I believe.”
Tanak explains Hyundai engine issue
Ott Tanak says a battery issue was the reason for his Hyundai failing to fire up at parc ferme this morning, but it is not related to the engine issues that followed.
The 2019 world champion admitted the engine problem is similar to what he battled through on Thursday night.
“This in the morning [at parc ferme] was a battery thing, we had no power, but it was not related to the engine issues later,” Tanak told Autosport.
“It is hard to say but somehow on our car it is quite difficult to manage the engine, the ALS [anti-lag system] is making the life a bit tricky.
“Everything is a bit on the hot side but when the stages are faster, we are okay.
“It is a tricky topic as they [the team] don’t really know why it is as bad as it is at the moment, but I don’t know what is in their plan. It seems as though there is no straightforward solution.”
