"I'm going to remain positive: these three years here have been absolutely incredible - it's been an ideal preparation for me", he said.
Sainz praised the team where he served his F1 apprenticeship and matured as a bona fide Grand Prix driver.
The internet was aware that Palmer's days were numbered with Renault even before the driver himself knew, and now it's official: Joylon Palmer's last race with Renault (and likely his last F1 race, period) will be in Sunday's Japanese Grand Prix.
Sainz had already signed a deal to race for Renault next season but the Spaniard is being released from Toro Rosso four races early as "circumstances have changed".
Palmer's exit means a return to Toro Rosso for Russian Daniil Kvyat, who was recently axed in favour of French rookie Pierre Gasly.
Sainz said competing in the final four races of the season would help with his preparation ahead of his full-season debut with the team in 2018.
"When I'm heading back on the plane maybe it will sink in that I'm not driving for Renault again and maybe not in Formula 1 again".
"The driver I am today, compared to the driver I was in 2015, is a more complete and better one, and this is thanks to Toro Rosso, who are a great group of professionals".
Cyril Abiteboul, Renault Sport Racing Managing Director, added: "I would like to thank Jolyon for his commitment to the team and his professionalism".
"We look forward to beating him on track".
"I've been with them for three years and it's a shame not to be going to Austin with them".
Regarding his race through the field, Palmer stated; "From 18th on the grid, I think finishing 12th is about as good as we could of done, apart from if we didn't have the VSC at the end [deployed for Lance Stroll's stricken Williams] and had one more lap".
"We will go our separate ways and I will work on something else for my future".
The announcement from Toro Rosso also confirmed Gasly for the USGP, meaning he will not fight for the Super Formula title in Japan despite sitting just half a point behind the leader heading into the season finale. Therefore, on behalf of everyone at Toro Rosso, I wish him all the very best with his new team.
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