Thursday 4 February at 08:31 : Feb.4 (GMM) World champion Jenson Button on Wednesday had a "frustrating" opening day at the wheel of his 2010 mount.

The Briton's first try of the new McLaren, because he is 7cm taller than his teammate Lewis Hamilton, was delayed due to difficulties with his seating position, and when he did hit the Ricardo Tormo circuit he was still sitting too high.

He then finished as just the tenth quickest driver of the three-day schedule, and markedly off teammate Hamilton's pace in the same MP4-25.

"This test was always to get used to the environment inside the cockpit, get used to the team and to run through all of the checks you do at the first test," said the 30-year-old.

He declined to say that McLaren's new car is a "great" one.

"There is a lot more to get my head round here, to work on, before I would (say that)," said Button.

Button also played down Ferrari's dominance of the Valencia test.

"Ferrari are fast but we don't know how fast. You never know what fuel they are running when they do those times," he said.

Fernando Alonso, having set the fastest time of the test in front of a record crowd of more than 36,000, also played down the obvious speed of the F10.

"I'm happy to be in the car, but I remain cautious," he said.

"I think maybe if we topped the times (on all) the three days (it) is related to fuel load or whatever," Alonso added. "But I found the car easy to drive and I feel good."

The Spanish media then quoted him as saying ominously that he did not extract "100 per cent" of the car's performance at Valencia.