Valtteri Bottas has admitted that the Williams might have trouble staying with closest championship rival Force India in this weekend's Singapore Grand Prix after trailing the Silverstone-based team on the opening day of practice.

Bottas and soon-to-retire team-mate Felipe Massa managed only 15th and 14th positions respectively on the Friday night timesheet and, while admitting that there was definitely pace to be found overnight, conceded that both Force Indias were a lot further ahead than expected.

"It is a bit more than what we were hoping for," the Finn said of the 1.7secs gap to Nico Hulkenberg on the FP2 list, "Force India was quicker than we expected so it will be challenging weekend for us, that's for sure.

"I wasn't quite happy with the car today. The track felt good, but the car didn't, so we have a lot of set-up work to do before tomorrow. But we've been in this position before and recovered well. If we can improve [the car], then I can also get more lap time out of myself and close the gap."

With the two Williams team-mates running contrasting programmes in a bid to cover as much ground as possible in the two 90-minute sessions, their converging performance was both good news and bad, but Bottas tried to keep an optimistic outlook for the rest of the weekend as the Grove team attempts to defend a thinning three-point advantage over its rival.

"[Felipe and I] were trying different things the all day and, also, in the night session, we ran different tyre compounds, so it's difficult to compare our lap times," the Finn noted, "We need to see everything really well before we decide how to approach the rest of the weekend.

"I had some traffic on the out-lap, so it was definitely tricky to get the tyres to work. I aborted the first timed lap and, when I then went for a quick lap, I felt I didn't get the tyres at their peak any more, although they were getting better all the time.

"We struggled quite a bit, the whole day, so it was certainly not a good day. However, the long runs didn't feel too bad and I have to say I was expecting worse tyre degradation. I didn't do that many laps with the ultrasoft tyres but, with the softs, I got a decent run in and it wasn't too bad. Now we just we just need to work to find the speed for qualifying, as grid position is very important here."