"There is - and were - options in Formula 1," said Magnussen. "I had options to stay in Formula 1 but I miss winning and I miss really being competitive and that has played a big part in how I look and pursue my future.
"I wouldn't stay in Formula 1 at any costs. I want to be competitive and I want to feel like I am going somewhere with my career, that I am progressing.
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"I have spent seven years effectively in Formula 1 trying to pursue success and I feel I have to complete that. I don't want to take a step back.
"There were options but I think there were options outside Formula 1 which were interesting."
When asked if he would contemplate taking on a reserve driver role, Magnussen, who filled such a position in 2015 with McLaren, refused to rule out the possibility.
"It depends. I don't know," he added. "I have had a year as a reserve driver and I wasn't racing anything besides my reserve driver role in 2015, when I was McLaren's reserve driver and that was tough.
"I am a race driver and I need to be racing. I have to be racing to stay motivated and happy. That would be first priority, to get a competitive race drive somewhere."
Grosjean, who completed a similar spell as third driver for Renault in 2009, pointed to the fact a reserve role at a competitive team could provide opportunities in the future, though insisted it was "not my preferred option".
"I don't rule it out," said Grosjean. "With the current situation, depending on the team you are reserve for. You may have the chance to have a good race so I think it is not a no, but it is something we need to think hard about."
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