MADRID, Spain - Michael Schumacher enters the last season of his three-year Formula 1 comeback with plenty of questions unanswered, while expectations for his Mercedes team remain muted after a series of disappointing results for the seven-times World champion.Schumacher's decision to come out of retirement in December 2009 was initially greeted with euphoria as he became another former champion on the F1 grid, which will feature six champs at the season-opening Australian Grand Prix on March 18.VETTEL FAVOURITESchumacher's return has thus far failed to live up to expectations though he holds several F1 records including the most World championships as well as most wins (91) and pole positions (68). However, in the two seasons since joining Mercedes when he looked to follow Niki Lauda and Alain Prost as drivers who came out of retirement to win F1 titles, his best performance was fourth at the Canadian GP.The 20-year veteran played down expectations as Red Bull's Sebastian Vettel heads into the new season as firm favorite to win a third straight title on a highly competitive grid that also includes former champions Fernando Alonso, Lewis Hamilton, Jenson Button and Kimi Raikkonen.Schumacher said: "Victory to me is unlikely. I see Red Bull in front. Behind them it will be tight, and you can be either hero or zero very quickly. It will come down a lot to the ability of how well you will be setting up your car to the respective track."Schumacher joined Mercedes after the German manufacturer bought out Brawn, F1's then-reigning Constructors' champion. He was reunited with Ross Brawn, his technical director at Ferrari where he won five of his titles and was joined by promising driver Nico Rosberg.WASTED POTENTIAL Brawn said: "Both are capable of winning. There's no doubt in my mind about that. We have to produce the car. I don't think we have the car yet but it has been a good step over where we were 12 months ago."The script hasn't gone to plan as Schumacher regularly finished in the top ten but never challenged front-runners Red Bull, McLaren or Ferrari over the past two seasons.Still, Schumacher was upbeat about his W03 car's reliability for the coming season:"There is potential. I do not have a clear picture yet. What we can say is that the gap between several teams will not be as big as it was in 2011."Whether Mercedes has done enough to propel Schumacher into a fairy-tale ending to his career is uncertain - though unlikely. Whether the season-ending Brazilian GP will be his last race is another unknown.Schumacher said: "That's just totally unimportant at the moment, I don't even think about it now. Now, it is all about focussing and concentrating on understanding our car, so that we will have a positive season. I will certainly not be nailed down now."