Valtteri Bottas believes Williams' performance in Singapore suggests it can challenge Mercedes for pole position at the Japanese Grand Prix.

After a difficult Friday, Williams improved dramatically on Saturday in Singapore and both Bottas and Felipe Massa were in contention for pole position during qualifying. With Singapore a track that wasn't expected to suit the Williams car, Bottas says there should be an opportunity for a repeat performance at Suzuka this weekend.

"Well if we could get so close in Singapore I don't see any reason why not here," Bottas said. "That would be nice. The gap was quite small and with the improvements we should be aiming to be as close to them as possible."

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And Bottas believes Williams has been closing the gap to Mercedes in the second half of the season after such a dominant start from Lewis Hamilton and Nico Rosberg.

"It looks a bit like that. We've been improving the car really step-by-step; only small things but continuously race-by-race. I think it is having an effect and now with a few more parts, definitely, I really hope so."

However, Bottas doesn't expect Williams to be the main challenger to Mercedes, saying Red Bull is likely to be particularly strong.

"Red Bull I think here is going to be more tough to beat. They have a lot of downforce and I think they're going to gain a lot in the high speed corners. But some races like Austin - with really long straights - they might struggle quite a bit and then we may have a really good chance to fight with them. Who knows? If all the updates we have here work well maybe we can challenge in other places as well."