The Citroen driver is now 1m06.7s ahead of Skoda's Jan Kopecky.
Kubica suffered his biggest moment so far on the day's final test, when he hit a house.
"I think house is OK and our car is OK!" he told AUTOSPORT.
"We came up a very narrow road and into a very narrow junction. I pushed the handbrake, the car turned and we clipped something."
He reversed and dropped 10 seconds, but was still quickest after an incredible debut day in the DS3.
"It's been a good day," he added. "To be honest, we were not really thinking of results. We took everything that came and it turned out to be a good day for us.
"We had lots of lessons today, it's been very productive."
Kopecky had hoped drier conditions might allow him to close the gap, but admitted there was nothing he could do, enduring a monster moment on SS7 as he tried.
"We were really flat-out," said Kopecky, "really trying. Then one kilometre before the finish we had a huge moment - we were sliding for more than 40 metres.
"We slowed down a bit after that."
Peugeot driver Craig Breen is secure in third place, but the Irishman was left ruing an unsuccessful suspension change on his 207.
"The car wasn't quite so precise," he said, "we've had a bit more understeer in those last two stages. That wasn't the plan."
Breen's team-mate Jeremi Ancian was fourth, with local hero and Mini RRC driver Luis Monzon fifth.
Andreas Aigner continues to lead the Production Car Cup in his Subaru Impreza after a trouble-free run through the afternoon.
Behind the sixth-placed Austrian, there was drama in the 2WD championship. Joan Vinyes headed former Intercontinental Rally Challenge champion Enrique Garcia Ojeda, but failed to make it back to service after the alternator belt broke on his Suzuki.
Garcia Ojeda takes over at the top of the two- wheel drive standings.
Leading positions after SS8:Pos Driver Team/Car Time/Gap 1. Robert Kubica Citroen 1h29m32.0s 2. Jan Kopecky Skoda + 1m06.7s 3. Craig Breen Sainteloc Peugeot + 1m51.0s 4. Jeremi Ancian Sainteloc Peugeot + 2m43.8s 5. Luis Monzon Canarias Mini + 3m36.2s 6. Andreas Aigner Stohl Subaru + 4m50.3s 7. Enrique Garcia Autogomas Citroen + 6m01.8s 8. Germain Bonnefis Renault + 6m17.4s 9. Joan Vinyes Iberica Suzuki + 6m39.5s 10. Gorka Antxustegi Iberica Suzuki + 6m40.1s
The fifth Intercontinental Rally Challenge was the best yet, and it was Skoda UK driver Andreas Mikkelsen who emerged as champion from a tense five-way shootout at the season finale. Matt Beer reviews the season