* Hamilton crashes on first day of testing with Mercedes
* Team have another troubled day (Adds Hamilton quotes)
By Alan Baldwin
JEREZ, Spain, Feb 6 (Reuters) - Lewis Hamilton escaped unscathed from a high-speed scare that brought a premature end to his Formula One test debut with Mercedes on Wednesday.
The 2008 world champion was doing about 300kph at the end of the straight when his rear brakes failed and pitched his car off the track, across the gravel and head-on into the tyre wall at the Jerez circuit.
Mercedes put the impact, after just 15 laps and only an hour and 48 minutes into the morning session, at around 60kph and the damage to the front suspension was enough to rule out any further track time for the day.
"I hit the brake, and for a split second it was working, but then the pedal just went straight down. It wouldn't work any more. Then I just had to brace for impact," the 28-year-old former McLaren driver told reporters.
Asked if there were still some positives from the day, he smiled and replied: "Definitely. I'm alive."
The crash, at the Dry Sack corner made notorious in 1997 when title rivals Michael Schumacher and Jacques Villeneuve collided there in the decisive 1997 European Grand Prix, compounded a difficult start to testing for Mercedes.
The British-based team are already under pressure to succeed after a lacklustre three years with now-retired seven times champion Michael Schumacher and have signed Hamilton to spearhead their challenge.
Their first pre-season test of the new car had already got off to a difficult start on Tuesday when an electrical problem and brief flare-up sidelined Germany's Nico Rosberg for the whole day after just 14 laps.
The problems were not connected but put Mercedes, with principal Ross Brawn's position at the helm already being questioned after management changes, even more under the spotlight.
"Lewis suffered a loss of rear brake pressure, the front brakes enabled him to slow the car but he couldn't avoid the barrier," said Mercedes, who said the problem was caused by the hydraulic brake line connecting to the right-rear caliper.
Hamilton, who spent the rest of the afternoon in briefings and watching his rivals put mileage on their new cars, conceded it had been a "bit of a slow day" but refused to be downcast.
"This is what testing is all about," he said. "What's happened is disappointing for all of us in the team because everyone has worked so hard over the winter, and we didn't anticipate this.
"I'm glad we get it out of the way now so we don't have to worry about it in the future.
Hamilton said he had taken "a bit of a thump" but the experience was not new and nothing like the crash he had at the 2007 European Grand Prix at the Nuerburgring when his McLaren lost a wheel.
"That was a lot more painful, but I got back in the car and got on with it. I've had lots of crashes in my career and I've been able to bounce back...so I'm not fazed," said the Briton, who will be back in the car on Friday.
"I've been racing for seven years, so it is not like I am lacking in experience, so I am pretty relaxed about it."
The Briton is opening a new chapter in his Formula One career with Mercedes after six years in F1 with regular title contenders McLaren, the team that had backed him from his early teenage years in karting.
Mercedes have one win, last year's Chinese Grand Prix with Rosberg, to their credit since buying the championship-winning Brawn team in 2009.
On the plus side, Hamilton pointed out that Mercedes did not even unveil their car last year until the second pre-season test so they were already ahead of where they had been in 2012. His first impressions of the car were also positive.
"It started out quite well, we did quite a lot of installation laps to get through a few different processes, which all seemed to go really well," he said. (Editing by Clare Fallon and John Mehaffey)