Fernando Alonso has refused to rule out returning to Ferrari in 2018 - and warned McLaren they will have to become winners this year to persuade him to stay.
Alonso, whose contract with McLaren expires at the end of the year, will consider his future following the summer break and has stressed he will leave the team unless they convince him they are ready to be champions again.
"I like this Formula 1 and my intention and first priority next year is to race in Formula 1," Alonso told reporters at the Spanish GP about his future. "But not only to race but also to win.
"I am happy with McLaren but we are not winning. If, before September or October, l see a possibility that we are in a position to win in 2018, l will be more than happy to stay with the team.
"If that is not the case, l will be more than happy to talk to anyone."
Alonso has already been linked with another of his former teams, Renault, about a winter transfer, and added: "I am open to anything."
The Scuderia are expected to have a vacancy for next season as Kimi Raikkonen, currently struggling to keep pace with championship leader Sebastian Vettel, is out of contract at the end of the current campaign.
"I am ready to race whoever comes along," added Vettel. "But l am very happy with how we are set up now."
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One drastic option for McLaren would be to drop engine suppliers Honda and seek a customer deal with either Mercedes, Ferrari or Renault. But any such divorce would be financially grievous for the team and Alonso is convinced that the partnership - whose reunion lured him back to McLaren two years ago after five seasons at Ferrari - will be given time to mature.
"McLaren and Honda have a long-term partnership with a very strong commitment from both," he said. "Now things are not going the way they thought at the beginning, but they are putting a lot of effort to improve the situation.
"So I think the McLaren-Honda name and partnership will be for the long term, so they need to find a way to extract performance and power from the engine. Hopefully we can see the progress in the very short term and this year fight for something big."
McLaren chief Zak Brown told Sky Sports last week that a return to Mercedes power was not on the cards, with Honda thought to be sourcing external help to try and resolve their long-term engine problems.
Asked if he would be sad if he had to leave McLaren in order to win in 2018, Alonso told Sky Sports News HQ: "Sure. With this project everyone is putting a lot of energy and commitment. The technical staff we have in the team now is at the top level and we have some of the best guys from all the teams.
"Unfortunately, we are underperforming on the power unit and we cannot do much about that. It's the third year of the project and we didn't see the progress we were hoping for. But the season is long, we are only in the fifth race, and we still have the potential to extract much more than what we are doing now.
"So I still expect a strong reaction from the team and in a couple of races time to be very competitive. If that happens we'll see what we can do in the future."
Alonso is yet to finish a race this season, never mind score a point, and his campaign plunged to a new low in Russia when his car broke down on the formation lap. Alonso, appearing at his home race this weekend ahead of a guest appearance in the Indy 500, described the embarrassment as "unacceptable" and made clear his patience is not so much wearing thin as exhausted.
"We have been underperforming for the first four races," he said. "We didn't finish any of the grand prix yet in my case, so the first step is to improve what we have been doing so far, which is not difficult.
"Second is to find how competitive we can be at this track compared to the last couple of races and with the upgrades everyone brings here normally things may change, so we need to find out where we are and which positions we can move into.
"Then reliability we need to raise our game and hopefully finish with both cars this race to accumulate some mileage. Hopefully this is the starting point of a new championship for us.
And, on the back of two consecutive non-starts for the team, Alonso added: "Really bad for us and, as a team, not being able to start the race with Stoffel's car in Bahrain and with mine in Russia is completely unacceptable. So we need to raise our game here in terms of reliability and hopefully finish with both cars."
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