Last week Alonso revealed he had been approached by Red Bull with an offer for next season.
“I had other offers as well,” he told Autosport. “I don’t think my targets or my challenges for next year were in F1 anymore.
“The offers I had, including that one [from Red Bull], they were not for winning.
“They are [all] probably, on performance, more than one second from pole position, as we saw [in Belgian GP qualifying].
“To be fifth or sixth or seventh will not be the same challenge or enthusiasm as I can find away from F1 in 2019.
“I made my decision a couple of months ago and I’m extremely happy with that.”
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