Michael Schumacher admits he has been surprised by the performance of Mercedes this season, given the struggles the team faced in 2012.

The Brackley-based team trailed in fifth in the championship last year having picked up just 142 points, 66 points less than it already has on the board this time around.

Schumacher himself scored just 49 points to end up 13th in the drivers' championship, and elected to retire for a second time at the end of the year - with Lewis Hamilton coming in as his replacement.

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Already this season, the team has now won three times and sits second to Red Bull in the championship standings, something Schumacher admitted was a surprise given the way that 2012 had panned out but said he was pleased that former team-mate Nico Rosberg was now being able to show what he could do on a regular basis.

"Given where we finished last season, it is a surprise to see them where they are now," he told Eurosport. "I couldn't see that coming, but I'm happy for them.

"People probably also understand a bit better now how good Rosberg is."

Prior to confirming he would walk away from F1 for a second time, speculation had linked Schumacher with a move elsewhere on the grid to extend his career still further, but the seven-time champion said he didn't regret his decision.

"The sport is about more than just the 20 races," he said. "Through the whole season you have to work very hard for the team and there is a lot more to do to make sure you are ready.

"In the three years I was back for, so much energy was consumed that I would not have been able to continue at the level I want to be at to measure myself.

"It would not have been enough for me to win a few races as my aim was to fight for the championship."