SEPANG, Malaysia - Severe tyre degradation on Friday caused by the searing Sepang temperatures shrank Mercedes' advantage over Formula 1 rivals during practice ahead of the 2014 Malaysian GP.Mercedes' driver Nico Rosberg was again quickest in practice but only by 0.2sec over the top six cars when track temperature peaked at more than 50 degrees, played havoc with tyres and caused a number of uncharacteristic spins.Hamilton, quickest in first practice, spun off in both 90-minute sessions. He later said the track was like "driving on marbles". Fernando Alonso also got into trouble with his Ferrari.SAUNTERED TO THE WINRosberg's team was surprised by the excessive degradation despite the heavy braking the track demands. His best (the top) time was 1min39.909 - which was pretty darn quick considering the fastest time from the 2013 race weekend was by Sergio Pérez and his McLaren - 1min39.199.The driver himself said: "It was a decent day, tough conditions out there really, very, very hot and tough for the car, tough for the tyres even. We never expected so much tyre degradation."It's a big challenge."Rosberg had sauntered to a 25sec win in the opening race in Melbourne two weeks earlier and although he was again quickest in Malaysia practice, the difference was a mere 0.035sec over Ferrari's Kimi Raikkonen.Asked whether tyre wear would lead to a different race in Sepang Rosberg said the field had been helped by the degradation issues but he still expected a close fight with his team mate for pole."There will be lots of strategies and with the tyres anything could happen," he opined. "It will always be close, we just need to try to stay ahead. Being on pole, important as it is, just getting that clean start away from the front is always a big advantage."'IT'S THE SAME EVERY TIME'Melbourne pole-starter Hamilton, however, said the tyre troubles did not suprise him. In a guarded discussion with reporters the 2008 champion said he was more focused on getting his car around all 56 laps of Sunday's race having been forced to quit after only three at Albert Park."It's the same every time I come here, it's to be expected. It's too hot for the tyres, they are not working very well in these conditions. Even though we've got the hard tyre, the tyres do not feel great - but that's the same for everyone."Hamilton was fourth-quickest overall on Friday with 2013 champion Sebastian Vettel ahead of him after 30 laps, despite concerns about a faulty fuel-sensor and requiring a new electronic power supply for the weekend.Alonso was fifth and his former team mate Felipe Massa was sixth for Williams."I never do (look at Friday's times), it's good that everyone is close, what matters is what happens tomorrow," Hamilton said, grateful that his repaired engine from Melbourne made it through unscathed."It saves us perhaps a grid penalty further along the way, so it's good, I'm glad that I had that less mileage on the engine so I hope I can push it a bit more."