Williams technical chief Paddy Lowe has conceded Formula 1 rival Force India outdeveloped his outfit this season, after losing out in the battle for fourth.
Last year, Williams was ahead of Force India in fourth until Italy before being leapfrogged and falling back - but in 2017, Force India pulled away considerably earlier and Williams could not respond.
Retiring Felipe Massa has raced for Williams continuously since 2014 and said earlier this year that the team has not been able to develop its cars as expected for any of the past campaigns.
When asked where he felt Williams had lost out to Force India this term, Lowe said: "We started the season with a car that was generally quicker than theirs. We've ended generally slower.
"So we have lost ground to Force India on performance. We have both been developing through the season at a rate you don't see - and they've done a better job.
"It's our job to recover that and more over the winter. We need to make a bigger step with the new car than they make.
"Renault have also come through with better performance through the season."
Lowe also admitted Williams' didn't take its opportunities earlier in the year, particularly on rookie Lance Stroll's side of the garage.
While Massa scored in four of the first six races, Stroll retired from the first three races and did not score a point until the seventh race of the season in Canada.
"The first half of the season, when we had quite often the quicker car, we weren't scoring the points particularly on Lance's side, when the opportunities were presented," said Lowe.
"With Lance new to the sport this year, it was always going to be a more difficult first half than second half. Those two factors were enough for Force India to get a stronger tally."