May 18 (GMM/ Niki Lauda says he always knew 2015 would be a tougher challenge for Mercedes.

The German outfit utterly dominated last year, but so far in 2015 Ferrari has put up a fight.

“The higher you climb,” the F1 legend and team chairman told Kolner Express newspaper, “the thinner the air.”

Barcelona winner Nico Rosberg, however, said he is not expecting any other teams to intervene in Mercedes’ title push.

“No,” he told Welt am Sonntag newspaper. “Just us and Ferrari.”

Lauda continued: “It quickly became clear to us that Ferrari had found 40 horse power more compared to last year.

“So we had to develop faster as well in order to maintain our competitive edge.”

The strategy appeared to have worked in Spain recently, as Mercedes’ Rosberg and Lewis Hamilton easily finished first and second.

Lauda said: “The victory in Barcelona was important because I know from experience that whoever is quick there is quick everywhere.”

The great Austrian said he is also particularly happy for the pole and race winner Rosberg.

“Until Barcelona, Lewis was driving like an extra-terrestrial, and I thought it would be impossible to beat him. But Nico recovered and managed to break Lewis’ run.

“That’s good news for him and also for us,” said Lauda.

Rosberg has won the past two Monaco grands prix, but Hamilton will be determined to resume his run of top form this weekend in the Principality, Lauda warned.

“He (Rosberg) has to go there and be on pole again,” he said, “because if Lewis is on pole, it’s over.”