By Patrick Johnston
SINGAPORE (Reuters) - Formula One world championship leader Lewis Hamilton was unhappy with track changes at the Singapore Grand Prix on Friday as he and his Mercedes team mate Nico Rosberg endured an uncharacteristically tough day in practice.
The Briton, who holds a 53 point lead over Rosberg with seven races remaining, was fourth fastest in second practice around the Marina Bay Street Circuit with the German down in the seventh as Red Bull's Daniil Kvyat topped the timesheets.
The Russian's efforts of one minute 46.142 put him ahead of Ferrari's Kimi Raikkonen with the Red Bull of Daniel Ricciardo third and Hamilton 0.337 seconds behind Kvyat.
Hamilton, reigning world champion, said there was nothing wrong with his car which has dominated all this year but did query the decision by organizers to tighten turns 11 and 12 and widen the 13th.
"I don't like it, it made the track worse, the track was fine the way it was before," he told reporters as he continually wiped sweat from his brow.
The changes were made to help increase overtaking opportunities on the narrow street circuit which has seen five pole sitters take the checkered flag in seven previous editions of the race.
Hamilton has taken 11 poles in 12 races this season but acknowledged it would be tough to claim another on Saturday after a strong showing by Red Bull and Ferrari on a circuit where, he said, "you can't overtake for the life of you."
"Definitely we have a genuine challenge from these guys and I'm looking forward to seeing how it goes," the 30-year-old said.
Rosberg had finished top of the time sheets in first practice ahead of Hamilton but finished behind both Ferraris, Red Bulls and Force India's Sergio Perez in the later one.
"We didn't do a good job today and the others have been pretty quick...but I learnt a lot and it will be OK by tomorrow," he said after finishing 0.639 seconds back of Kvyat.
The German, who was forced to retire early here last year, said he would be working on a way to limit pit stops to aid his race bid.
"Tyre wear is quite high so it is difficult to do two stops, that's going to be the key to the race so that's what we need to work on tonight."
The disappointing display, by the team's sky high standards, drew short shift from Mercedes Motorsport head Toto Wolff.
"We are not happy with the performance today, we couldn’t get the tyre to work the way we wanted on the one-lap performance or the long runs either," he said.
"We some spectacular laps from the Red Bulls so we just need to get our heads together and assess what happened."
(Editing by Alan Baldwin)