BERLIN, Germany - Mercedes boss Dieter Zetsche insists Michael Schumacher's disappointing return to F1 was due to an under-performing car and not the driver.Schumacher, 43, returned to Formula 1 in 2010 after a four-year hiatus but failed to make an impact on the World Drivers' championships.CARS NOT GOOD ENOUGHZetsche told German daily Bild: "We have not been able to provide both drivers (Schumacher and team mate Nico Rosberg) in the past three years with a car that was able to win most races. "With the engine we were competitive, but not with the car. And it would be completely wrong to reproach either Nico Rosberg or Schumacher."Mercedes will not extend Schumacher's contract, which finishes at the end of the 2012 season; he'll be replaced by Briton Lewis Hamilton.Zetsche also insisted Schumacher's age was not a factor; Argentina's Juan-Manuel Fangio was World champion at 46 during the 1950's.HIGH EXPECTATIONSThe Mercedes boss admitted that "given the name of Michael Schumacher, expectations were probably too high". "But I called Michael on the day the decision was announced to speak with him. It was a very good discussion," Zetsche said.Schumacher won the most recent of his seven Driver's titles in 2004 with Ferrari and finished eighth in 2011 with Mercedes fourth in the Constructors' championships.He started in F1 in 1991, has raced in more than 300 GP's and won 91 - but none since his return.