Daniel Ricciardo believes Sebastian Vettel has been revitalised by his jump from Red Bull to Ferrari, and backed the four-time world champion to be one of Mercedes' fiercest rivals in 2016.
Vettel was out-classed by Ricciardo when the pair were team mates at Red Bull in 2014, but was resurgent this season as he won three times and comfortably had the edge on new stablemate Kimi Raikkonen.
Asked to name his stand-out drivers in 2015, Ricciardo said: "Seb picked himself up from - by his standards - not a great 2014. He found a new lease of energy and life at Ferrari, and ran with that opportunity.
"Mercedes are still the team to beat [in 2016], I'm sure of that. Ferrari are interesting - they made a big step this year, and if they make another step like that they'll be in good shape to fight with Mercedes."
Race winner Sebastian Vettel (GER) Ferrari and second placed Daniel Ricciardo (AUS) Red Bull Racing celebrate in parc ferme at Formula One World Championship, Rd13, Singapore Grand Prix, Race, Marina Bay Street Circuit, Singapore, Sunday 20 September 2...
Seb picked himself up from - by his standards - not a great 2014 - he found a new lease of energy at Ferrari
Daniel Ricciardo
Asked about his own season, and Red Bull's prospects for next year, Ricciardo added: "I look back with a smile. Sure it had some frustrations but there were still some positives. I feel I learned a lot. I'm still young, I feel I still have a lot to learn and achieve in the sport. Some of the harder times, I took a lot out of.
"Hopefully we can find a few more occassions to battle [at the front] - we can definitely make progress, and I definitely enjoy fighting at the front."
Ricciardo also predicted a renewed challenge within Red Bull from his 21-year-old Russian team mate Daniil Kvyat, who actually had the advantage in this year's championship fight, scoring 95 points to the Australian's 92.
"I think Dany had a solid season; it started off a bit slow but he picked himself up well and maintained that," Ricciardo commented. "He started to get more comfortable in the team and felt a bit more established.
"Obviously it will keep coming with age, so next year he will be more competitive and I'm sure he will find more speed again. A new challenge for me, but I'm looking forward to that.
"I know if I put in the off-season I need and have my head in the right space, which 99.9 percent of the time it is, I think it will be a fun season."
The 2015 campaign was actually Red Bull's worst for almost a decade - they failed to win for the first time since 2008, claimed just three podiums, and finished fourth in the constructors' championship.
