Williams tech boss Paddy Lowe says McLaren's new partnership with Renault is a cause of concern for the Grove-based outfit in 2018.
After putting an end to its fruitless three-year relationship with Honda, McLaren finds itself on the verge of a new dawn with engine partner Renault.
And a potential resurgence this year of F1's second most successful team has put Williams on alert.
"Honda have looked reasonably strong towards the end of the season so it'll be interesting to see what kind of step or not McLaren make over the winter.
"They are a competitor to be respected, so we've certainly got our eye on them."
In 2017, Williams failed in its quest to overhaul Force India in the Constructors' standings, but Lowe, who initiated many process and organisational changes at Grove after moving from Mercedes to Williams, was generally pleased with the team's performance and development programme in the latter part of the season.
"Getting the best out of what you've got requires incredible teamwork, whether it's a winning car or a midfield car," said 55-year-old engineer.
"We've had some losses, some errors, but overall the team has done a great job to pull that together. We would have liked to have been fourth.
"All credit to Force India, they have done a fantastic job, they deserve the position they've had.
"But holding on to fifth when you look round at the competition is not to be taken for granted.
"Particularly in the last third of the season, you won't know what we've done but there is quite a lot of lap time we've brought to the car just to stand still and hold on to that place.
"It's a really good performance programme we've had."