The McLaren driver scored points in the first five races, but he then had six abandonments and also produced some lacklustre performances.
"I didn't even have a chance to fight in the last races," he said.
Interlagos had given him some good memories, and what he called "several media moments".
In 2015, at the end of a poor season for McLaren, the Spaniard decided to relax after his car broke down during qualifying in Sao Paulo, sitting on a chair at the side of the track and sun-bathing.
This week, a similar chair filled with signatures was placed next to the paddock area where Alonso spoke to journalists.
"Keep the best F1 moments in mind," said a message from 'Sebastian', although it was unclear if it came from Ferrari's Vettel.
"Enjoy your retirement watching Disney videos," another message read.
At the end of that same 2015 Brazil race, Alonso and his then teammate Jenson Button climbed on to the podium. They had finished 15th and 14th respectively.
"Jenson and I walked next to the podium and we realised we weren't going to be there anytime soon. So we decided it was a good idea to take a picture," Alonso said, referring to the fact his previous podium had dated back to the 2014 Hungarian Grand Prix where he finished second.
The McLaren driver, once considered the most talented on the grid, has yet to reveal what the future holds for him, but McLaren's sporting director Gil de Ferran said Alonso had been talking to him about the IndyCar series.
"I told him what is good and what is bad there," De Ferran explained. "But moving there is a very personal decision."
Last year, Alonso skipped the Monaco Grand Prix to race at the Indianapolis 500, driving for Andretti Autosport in a McLaren-branded car. His car broke down when he was seventh and there were 21 laps to go, but fans still gave him a standing ovation.
- AP