Valtteri Bottas believes that Mercedes and Ferrari have pulled clear of Williams, after the team had initial optimism of challenging towards the front of the pack in 2016.
Williams has finished third across the past two years, having beaten Ferrari in 2014 and Red Bull in 2015, but currently occupies fourth in the standings.
Bottas accepts that the leading pair of manufacturers have edged away from Williams and that Red Bull and Toro Rosso are now in the mix with the Grove-based outfit.
"It seems that it’s going to be a really tough year, very close," he said ahead of this weekend's Chinese Grand Prix.
"Ferrari and Mercedes are definitely further away than the last two years so that’s unfortunate but we just need to keep trying.
"I think after that we are in that field with a few teams, Red Bull and Toro Rosso they seem to be very close, every bit we improve this season will make a big difference."