Lewis Hamilton admits he has work to do with his Mercedes car after a tough second practice session in Shanghai, although Valtteri Bottas was in impressive form once again.

The Mercedes duo have led the way in the early stages of the new Formula One season, with Hamilton second to Bottas in Australia before the roles were reversed in Bahrain last time out.

And Bottas again suggested he is set to challenge at the Chinese Grand Prix following Friday's practice sessions, shading Ferrari's Sebastian Vettel by 0.027 seconds in FP2.

Vettel had led Hamilton by 0.207secs to top the charts after FP1, but the Briton endured a tough time later in the day, unable to post a quick lap due to debris on the track as he ended up in fourth place.

He and Bottas had both spun earlier in the session on cold tyres, too.

"I was struggling with the car, so we've got work to do - particularly on my side, as Valtteri looked much more comfortable in the car," Hamilton said.

"We're going to work hard to find some tweaks and hopefully come back stronger [on Saturday]. The car has the pace in it to compete at the front, we just need to find the right set-up.

"It is nice to see how close it is between us, Red Bull and Ferrari. As we expected, the Ferraris still look quicker on the straights. But overall it's very close between the top cars."

Bottas insisted that he was also not satisfied, adding: "We still have room for improvement.

"But we know the others are going to work hard on their cars as well, so we're going to have to wait and see how much we can improve in relation to them."