|The new Mercedes appears unbreakable. They have not attempted any fast laps yet. They prefer to not show their hand until opening weekend.|
World champions Mercedes have now completed an incredible five race distances in two days of ominous pre-season testing in Barcelona.
Lewis Hamilton, the triple-world champion, racked up 156 laps on Monday before team-mate Nico Rosberg added a further 172 to the team's impressive overall tally.
And while it was Sebastian Vettel, the four-time world champion, who topped the order for the second successive day, it is Mercedes who have delivered a stark warning to their rivals ahead of the new season which starts next month in Australia.
"That was an F1 marathon today," said Rosberg after driving nearly 500 miles in his Mercedes W07. "It's great to see that we are already able to do a lot of mileage – and we didn't have a single problem either, which is really incredible.
"I'm very happy and proud of what the boys and girls at Brackley and Brixworth have done over the winter and I already feel very comfortable in the car."
Vettel, the Red Bull of Daniel Ricciardo and Force India driver Sergio Perez were all quicker than Rosberg here at the Circuit de Catalunya. But the trio produced their best laps with soft tires.
Rosberg posted his fastest lap, two seconds adrift of Vettel's best time with the durable, but slower, medium compound.
Rosberg added: "It's a big positive to see everyone pushing so hard in all areas – bringing new developments like the floor we saw today, for example.
"The opposition are chasing us hard and we need to keep being innovative to stay ahead. At this time it's not possible to say how quick we are compared to the others. We will maybe know more at the end of next week – but only Melbourne will give the full picture.
"Overall, it's been a great start to the testing program and I can't wait to be back in the car on Thursday."
Vettel also managed an impressive 126 laps before his Ferrari stopped on track in the closing moments of the session.
Elsewhere, Pascal Wehrlein turned in 71 laps for the dramatically improved Manor while new American outfit Haas continued their fine start to life in Formula One after Esteban Gutierrez completed 79 laps.
Fernando Alonso was ninth in the standings with 119 valuable laps under his belt, but Jolyon Palmer, the British rookie, completed the least mileage of all after encountering problems with his Renault.