By Alan Baldwin
SPIELBERG, Austria (Reuters) - Nico Rosberg's hopes of a third successive Austrian Grand Prix win took a hit on Saturday when he crashed in final practice and damaged his Mercedes' gearbox, incurring a five-place grid penalty.
Mercedes said mechanics were working flat out to repair the damaged car in time for afternoon qualifying at the Red Bull Ring circuit.
"A gearbox penalty will be incurred as a full new unit will be fitted for qualifying," a spokesman said.
Championship leader Rosberg is the only driver to have won in Austria since the race returned in 2014 after an 11-year absence and he is hoping to continue the run on Sunday to increase his 24-point lead over team mate Lewis Hamilton.
However the accident, after Rosberg had dominated Friday's practice sessions with the fastest lap at the scenic circuit nestling in the wooded Styrian hills, could hand the British triple world champion a significant advantage.
Mercedes said the car had suffered a left rear suspension failure after an unusually high load at full throttle on the exit kerb at turn two.
The car spun several times before spearing into the barriers.
The session was halted with 21 minutes remaining and the car brought back to the garage on a truck, with Rosberg, winner of five of the season's eight races to date, making his frustration evident on his return.
Hamilton had earlier expressed concern about his car's rear tires, asking the team over the radio to take a look because "something is not right."
"I'm going to go off if I keep on going," he said when asked to continue.
Ferrari's Sebastian Vettel, who will have a five-place grid penalty for an unscheduled gearbox change, lapped fastest in the session with team mate Kimi Raikkonen second on a bright morning with the threat of later rain.
Vettel's lap of one minute 07.098 was faster than Rosberg's Friday best. Hamilton, who struggled for pace in the first two sessions, was third in 1:07.308.
Australian Daniel Ricciardo was fourth fastest, ahead of Red Bull team mate Max Verstappen and the two Williams of Valtteri Bottas and Felipe Massa.
(Editing by John O'Brien)