Agencia Estado quotes the Brazilian as admitting he and Bottas, the team's Toto Wolff-linked 2012 reserve driver, "are in the same battle" for a single race seat for next season.
"As soon as we saw that the battle was strong, we saw that we needed a plan B, because you can't wait until the end (of the year) to see whether or not you are somewhere," said Senna.
"Ideally, I'd like to continue at Williams, because I believe that continuity is important. But if I am not there, I'll need to be somewhere else."
Some media reports claim Bottas, 23, will surely be Venezuelan Pastor Maldonado's teammate in 2013.
Senna responded: "We are in the race with Bottas, but at the same time, I am negotiating with other places to ensure I have a spot next year."
The Agencia Estado news agency also quoted Senna as saying his mother Viviane, the sister of the late and great Ayrton Senna, had ruled out a switch to the "dangerous" oval-based Indycar series.