McLaren's Alonso believes Mercedes looks 'stronger than ever' in F1

While the world champion team has not yet topped the times in Spain, Lewis Hamilton and Nico Rosberg comfortably completed the most laps on Monday and Tuesday.

"The Mercedes domination has not finished," said McLaren driver Alonso.

"They are very, very strong, and they are doing some interesting days of testing.

"These first two days, with that amount of laps [328 in total] means you have very clever things.

"So when you add the potential they are stronger than ever, even more than the last two years."

Rosberg, who was fourth quickest on Tuesday having only used the medium-compound tyre, said it was obvious there is more to come from Mercedes.

"We've been quite successful at the moment, and then laptime-wise of course it's not realistic where we are," he said.

"We haven't shown our cards yet, we're still holding back.

"The car is quick, but of course the opposition also seems to be quick, so how quick we are nobody knows for now."

While Rosberg's comments suggest Mercedes will show a bit more of its performance before testing is complete, Alonso believes the true picture will only emerge at the first race.

"There is no crystal ball in Formula 1 that tells you in what direction the season will go after two weeks of testing," said Alonso.

"We need to wait until Australia when we are all on the same conditions, with the super-soft tyre, with low fuel, and then we will see.

"Even then we will have to wait for two or three races to see how the year will turn."