The announcement that Lando Norris will partner Carlos Sainz Jr at McLaren next season fills in a key piece of Formula 1's driver market puzzle, which is edging towards completion.

Norris was picked ahead of Mercedes junior Esteban Ocon, outgoing McLaren driver Stoffel Vandoorne, and Force India's Sergio Perez. Of those Norris defeated to earn his grand prix graduation, only Perez's place on the 2019 grid looks secure.

He claims to have a contract signed already and the expectation is he will remain at Force India to partner the incoming Lance Stroll, whose father Lawrence is now one of the team's main backers.

However, missing out on McLaren means Ocon has lost a major option while the axed Vandoorne is damaged goods after two miserable seasons in F1.

Mercedes has worked tirelessly to find a home for Ocon since his planned move to Renault was derailed by Daniel Ricciardo's surprise switch from Red Bull.

However, Williams is the only other Mercedes-powered team on the grid with a vacant seat and its financial needs mean an engine discount is unlikely to be enough to place Ocon there.

Williams appears the final hope of several drivers. Sergey Sirotkin brings funding and has impressed the team in his rookie season, but his fellow Russian Artem Markelov has been linked and reserve driver Robert Kubica still wants an F1 race return.

Mercedes junior George Russell has also targeted Williams but a 2019 F1 graduation now looks difficult for the Formula 2 championship leader, with others ahead in the queue.

Russell's and Ocon's Mercedes ties are limiting other teams' interest, but Mercedes is willing to drop its juniors if necessary.

That would immediately make them more appealing to Red Bull, whose junior team Toro Rosso has not confirmed either of its '19 drivers.

Daniil Kvyat is on course for a surprise third chance in the Red Bull stable, but the second seat is still up for grabs.

Brendon Hartley could be retained, but he does not share the attraction of an unattached Ocon or Russell, or even Vandoorne, who came to F1 with a stunning junior record.

Haas is also yet to name either of its '19 drivers. Kevin Magnussen is expected to stay, while Romain Grosjean has regained his form after a bad start to the season.

Charles Leclerc's Ferrari move is back on, eliminating him from the Haas picture, so Ocon and Vandoorne could fight for a place there.

Haas is not interested in a rookie, which makes Sauber a better option for Ferrari's other protege Antonio Giovinazzi as Ferrari has an option on one of the Swiss team's seats.

Vandoorne was run by Sauber team principal Frederic Vasseur's ART team to the 2015 GP2 title, and his manager is also involved in Sauber's ownership structure.

It is understood that the prospect of bringing the Belgian to Sauber has been discussed, however, long-time Sauber driver Marcus Ericsson is backed by a team director.

That means at least 10 drivers are in the mix to race in F1 next season, but only six seats realistically remain available.

F1 2019 line-ups

Confirmed

Mercedes: Lewis Hamilton, Valtteri Bottas

Red Bull: Max Verstappen, Pierre Gasly

Renault: Daniel Ricciardo, Nico Hulkenberg

McLaren: Carlos Sainz Jr, Lando Norris

Expected

Ferrari: Sebastian Vettel, Charles Leclerc/Kimi Raikkonen

Racing Point Force India: Lance Stroll, Sergio Perez

Haas: Kevin Magnussen, TBD

Toro Rosso: Daniil Kvyat, TBD

Seats up for grabs

Haas x1

Toro Rosso x1

Sauber x2

Williams x2