The 42-year-old Spaniard is signed to the end of 2024 but is already talked of as a possible successor to Hamilton when the seven-time champion joins Ferrari next year.
Speaking after the launch of Aston Martin's 2024 challenger, the veteran admitted that he's in a strong position if he wants that seat, and kept the door open to such a move.
"I am aware of my situation, which is unique," he told the media.
"There are only three world champions on the grid, and fast world champions, because in the past maybe there were some champions who were maybe not so committed to be fast. And I am probably the only one available for 2025. So I am in a good position.
"But at the same time, when I make the decision whether I want to keep racing in the future, the first and only talk I will have in the beginning will be with Aston Martin. That will be my only priority.
"But if we cannot reach an agreement and I want to commit to race in F1, I know I have a privileged position. I am probably attractive to other teams, the performance they saw last year, the commitment."
Few Formula 1 drivers have competed in their 40s in the modern era, although 1992 champion Nigel Mansell was a race winner at 41 in 1994.
Double world champion Alonso is the most experienced of all time and the oldest on the current grid, starting his 22nd season and 378th race in March. He told reporters ahead of Monday's launch of Aston Martin's AMR24 F1 car that he was fitter than ever.
"The numbers we achieve in all the physical tests that we do every season, they were the best ever this year," said the winner of 32 grands prix, who has added a nutritionist to his team.
He said he needed to prepare himself better than ever to be fully ready if he were to commit to a project for years to come.
"I will not drive a few more years in Formula 1 just to drive and to have fun. I am not that kind of driver, I'm not that kind of person," he said.
One of three active world champions but the only one still on the market for 2025, Alonso said he would talk first with Aston Martin if he decided to continue.
"A few years ago I would say that maybe 42 or 41 was the limit. Now after I saw myself last year motivated and performing well, I was thinking maybe that I can keep racing a few more years," he said.
"Now this winter I've been exceeding a little bit the expectations in terms of all the physical tests and everything that I did, so I would say that if you are motivated and if you want to commit you can drive maybe until 48 or 49 or even 50.
"But at the same time you have to give up everything in life. Formula 1 needs total dedication... and I gave my life for 24 years to this sport, which I'm happy and okay with.
"I can keep doing it for a few more years but I don't know if I will be racing until 50 with such a demanding calendar and things like that. Not for the ability, but because there are other things in life that I am curious (about)."
Aston Martin team boss Mike Krack recognised Hamilton's move had made the driver market more complicated but told reporters: "We love Fernando, we have a very good relationship... we would be delighted, honestly, to continue with Fernando into 2025 and the year after."