FILE PHOTO: Formula One F1 - Japanese Grand Prix - Suzuka Circuit, Suzuka, Japan - October 10, 2019 Williams' George Russell REUTERS/Issei Kato/File Photo

ABU DHABI - George Russell, the only Formula One driver without a point this year, swapped his tail-end Williams for a title-winning Mercedes on Wednesday and ended up fastest on the final day of post-season testing in Abu Dhabi.

The Mercedes-backed rookie did 145 laps of the Yas Marina circuit, the second most of the day, with a fastest lap of one minute 37.204 seconds - more than three seconds quicker than his time for Williams on Tuesday.

Ferrari's Charles Leclerc crashed, bringing out red flags, but was still second fastest.

Russell's time for the champions compared to Tuesday's best, set by regular Mercedes race driver Valtteri Bottas, of 1:37.124.

Russell outqualified departing team mate Robert Kubica 21-0 this season but finished last overall after the Pole managed to score a point in Germany - the former champions' only top-10 finish of the campaign.

Wednesday was his second stint for Mercedes this season, with the highly-rated 21-year-old testing in Bahrain last April.

"It's been a pleasure driving the championship-winning W10 today. We tested the 2020 tyres and I've learned a huge amount about them which I'm sure will help both myself and the team going into next season," said Russell.

Canadian Lance Stroll was third fastest for Racing Point.

When Formula One returns in 2020, there will be a record 22 races on the schedule - and renewed concerns about the demands being placed on those who keep the travelling circus on the road.

The schedule has lost Germany but is expanding with the addition of Vietnam and the return after a long absence of Dutch circuit Zandvoort.