Toto Wolff said Mercedes "owe it" to Sebastian Vettel to consider the four-time Formula One world champion if they have a seat to fill next season.

Vettel will leave Ferrari at the end of this season and his future is up in the air.

Silver Arrows duo Lewis Hamilton and Valtteri Bottas are only contracted until December, although talks with the defending champion are expected to be held once the season starts and the Finn is also tipped to stay on.

Mercedes boss Wolff said it would be remiss of the constructors champions to dismiss the prospect of signing Vettel, however, citing Nico Rosberg's sudden retirement in 2016 as proof that anything can happen.

He said: "It's not lip service, but we owe it to a four-time champion not to come out and say straight away, 'No'. You need to think about it.

"On the other side, we have a fantastic line-up and I've very happy with both our pilots and George [Russell], but you never know. One of them may decide he doesn't want to go racing anymore and suddenly you have a vacant spot.

"This is why I don't want to come out in June and say, 'No chance, Sebastian is not racing for us'.

"I wouldn't do it to him as a driver, to be that blunt, and on the otherwise side, I've seen black swans appearing when nobody suspected - remember Nico Rosberg.

"In that respect, we're just keeping our options open but of course concentrating our discussions with our current drivers."

Carlos Sainz was confirmed as Vettel's replacement and it was announced last month that Daniel Ricciardo will take the Spaniard's seat at McLaren.

Wolff did not expect deals to be confirmed before the first race of the campaign, which is due to start in Austria on July 5, and says Mercedes are keeping their options open.

"We were quite surprised with the early movements because you need to start the season with a driver that you know you're going to lose," the German added.

"It's very tricky to develop a car without having that driver be part of it, and every single driver needs to be a team player as well, and that becomes second priority in my opinion for the relevant guy. For us, we want to take our time.

"Our priority lies on the Mercedes drivers, Valtteri and Lewis, and then obviously George and the Williams situation we're looking at with interest. Beyond that, you do not discount any driver.

"This is why I said I don't want to discount Sebastian as a four-time world champion, because who knows what happens in the next months?

"If I had told you in January that we would not have any racing in the first half of the year, nobody would have believed it. So, in that respect, we are just keeping our options open at this stage."