Picture: Matthew Childs / Reuters.

Hockenheim, Germany - Formula One leader Lewis Hamilton is hoping rain will come to the rescue as a Hockenheim heatwave threatens to dash Mercedes's hopes of a home German Grand Prix win on Sunday.

The five-times world champion and Mercedes have been dominant this season, the team winning nine of 10 races so far, but they wilted in the sweltering heat at last month’s Austrian Grand Prix.

Mercedes has made changes to help cool the car but, with much of Europe caught in the grip of a heatwave, Hockenheim could be even hotter than the temperatures were at Spielberg where Max Verstappen won for Red Bull.

"If it stays this hot we are going to struggle and be in trouble," the 34-year-old Briton, who leads team-mate Valtteri Bottas by 39 points in the standings, told reporters on Thursday.

"There is not much we can do (when it comes to cooling), it is a much bigger design issue when it gets hot which is not so easy to change.

"We are working towards it but it is very small steps and small increments which are not making a massive difference," he added.

"It is definitely a good thing if it rains."

Friday, when the cars first take to the track for two sessions of practice, is expected to be just as hot as Thursday, when temperatures touched 39 degrees Celsius.

But thunderstorms are predicted for Saturday, when qualifying is held, while the chance of rain on race Sunday - a 200th Formula One start for Mercedes as a constructor - has also gone up.

Hamilton, who won in the rain from 14th on the grid last year even as Ferrari rival Sebastian Vettel crashed out of the lead, is renowned as a wet-weather specialist.

He has won the German Grand Prix four times, three of those victories at Hockenheim, and is chasing his eighth win of the season on Sunday.

Rain would cement his status as favourite to collect a fifth German Grand Prix win, a record for success at the race in the world championship era.

Hamilton, who won his home British Grand Prix at Silverstone for a record sixth time two weeks ago, is still spoiling for a fight.

"Last year here... I think Ferrari were slightly quicker but I hope it gets closer throughout the year," he said. "I’m always ready, it just has not always been the case every year.” 

Reuters