Michael Schumacher says McLaren was 'very strong' in the final day of testing at Barcelona, where Lewis Hamilton set the quickest time of the week.
But the Mercedes GP driver admits it is virtually impossible to know what the order will be in the first race of the season in Bahrain in two weeks' time.
"They looked very strong today, no doubt," Schumacher told reporters about McLaren at the end of the day.
"But then it depends on who is going to bring what to Bahrain. It's something we'll have to wait for to find out.
He added: "We have some improvement [for Bahrain], but is it much or is it not compared with what the others may or may not do? It's a puzzle that's difficult to read."
The seven-time champion claimed the mood at his team was positive ahead of the start of the season, as he feels things are starting to come together for the German squad.
"We are generally positive. It's hard work for everybody because you produce, you test, it's long nights for the mechanics.
"It's a lot of hard work and I'm thankful everybody is pushing like hell, both here and in the factory. So everybody is now looking forward to probably breathe a little bit.
"But just a little bit because preparation for Bahrain is short. In general we feel confident that our performance has come up, so slowly things come together."
Schumacher admitted he enjoyed the moment where Hamilton and him ran their own personal "race" during today's session, when both were running together during a long stint.
"It was fun," said Schumacher. "It was interesting to see whether overtaking was possible. Obviously it was not possible as we were in different stages of our runs and tyres. At one point I felt there was no point because I was harming my own situation a little bit.
"I basically gave way and basically let him do what he wanted to do and I could continue with what I wanted to do."