Former F1 world champion Nico Rosberg has praised the "phenomenal" turnaround completed by Mercedes.
Rosberg's former team started slowly in F1's new regulatory era, often finding itself in battles with the likes of Alpine, McLaren, Alfa Romeo and Haas rather than fighting for victories alongside Ferrari and world champions Red Bull.
But whilst the opening part of the year was hampered by porpoising issues, developments added to the W13 saw steady progress back towards the front, culminating in a victory for George Russell at the São Paulo Grand Prix in what was a one-two finish with Lewis Hamilton.
"It is an incredible turnaround by them. It is so phenomenal," Rosberg told Sky Sports' Any Driven Monday.
"They were sometimes up to one second off the pace at the beginning of the season from Ferrari and Red Bull, at least seven-to-eight tenths.
"Come to Mexico and Brazil, suddenly they are the fastest car out there in the race, that is an incredible turnaround.
"I think Abu Dhabi was a setback, being so far off in qualifying was a bit of a shock to them and a lot of it will come down to them struggling with drag on the straights."
Rosberg believes in Mercedes title tilt
The Abu Dhabi Grand Prix saw Mercedes take a step backwards ahead of the winter break, but 2016 champion Rosberg is still counting on the Silver Arrows to be title contenders next season.
"The car inherently has more drag than a Ferrari and a Red Bull so that is something that they still need to try and figure out," said Rosberg.
"That is the last key that is missing to really challenge for wins and championships next year.
"Are they able to sort it out over the next couple of months in the winter? My guess would be that they start a little bit behind still because they spent so much time figuring out the problems on the car rather than moving forward and developing the car so I think they will still be a little bit behind.
"But through the season, the way they have been developing they will have a chance to challenge for the championship, I do believe that."