Williams' head of performance engineering Rob Smedley has said he is deeply disappointed with how the Japanese Grand Prix turned out for the team.
"Not our best race. Not by a long shot. A lot of things we need to think about and improve upon. Without going into a great deal of details we didn't operate particularly well today.
"The car was a little bit slower than what we thought it would be going into the race - we're talking small margins, but it's enough that we weren't actually quicker than Ferrari but a couple of tenths slower.
"Overall I think it was a really disappointing day for the team."
Early in the race, Valtteri Bottas was running in third place behind Lewis Hamilton and Sebastian Vettel, but as the afternoon went on he found himself demoted to fifth after being passed on track by Nico Rosberg and jumped in the second round of pit stops by Kimi Raikkonen.
"We made the right decision with the first stop with Valtteri, pitting him to try to retain the position ahead of Rosberg. That was good, made them think a little bit," said Smedley.
"Rosberg actually passed Valtteri on track even though strategy-wise we'd put him out in front. The pace of the car compared to the Mercedes allowed Rosberg to get past on track and then we had Raikkonen who was sat behind.
"We just probably weren't quick enough to react to get that final set of tyres on and keep him behind as well," admitting that a slow second stop for Bottas had cost him the position.
"We have a fairly regular occurrence of one issue which is the wheel nuts being a little bit tight to take off which I think is reasonably well documented. But then today we had other things which were thrown at us as well.
"Part of that definitely helped Kimi to get in front of Valtteri as well. So it's something we need to address. We can't go racing like that with a tactical handicap."
Bottas' team mate Felipe Massa was never in the running after contact in the first corner left him with damage and forced an early pit stop.
"It's very difficult for us to tell what front wing damage we'd incurred at that point but he'd done a whole lap with a punctured tyre so we changed it for caution which turned out to be right because we'd broken one of the turning vanes on the front wing and then worn out all the right hand side footplate as well. Afterwards he had a little bit of damage to the floor as well that he carried through the race."
Although Williams have gained ground on Red Bull in the constructors championship, Smedley remained disappointed with the outcome at Suzuka.
"Quite happy we managed to claw ten points on them," he agreed. "Definitely in a race like today it's the primary thing we need to try and do.
"That is a positive, but it's not enough of a positive. They didn't score anything, so it was a race where we should have had 18 or 20 points on them, not ten.
"As usual, there's a tsunami of work to do after every race. This is perhaps a slightly bigger tsunami."
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