Ferrari's Kimi Raikkonen set the best time of anyone at the Circuit de Barcelona-Catalunya. His team mate Sebastian Vettel was almost a match for Mercedes' Valtteri Bottas in terms of mileage and reliability.
Of of which means that Hamilton is starting to wonder whether this year's world championship might not be the walk in the park for him that it once seemed.
"They have been doing a fantastic job and I think there's more to come from them," Hamilton said when asked by Motorsport.com for his thoughts on Ferrari's performance. "Their pace this week has been spectacular.
"Red Bull are still there," he added. "I don't know if they brought their upgrade package here this week but usually they bring it for the first race. I expect us to be having a real serious battle with both these teams."
At times it appeared that the Ferrari could have gone even quicker. Vettel backed off on his flying laps on Thursday and later denied that his team were favourites in 2017.
"I don't know if there are mind games going on," Hamilton said. "Sometimes there have been potentially some teams sandbagging in the past, but I don't really see that as being a benefit to any team.
"Every year in testing the goal is not necessarily to go as quick as you can. It's to find out what you have in the car."
Certainly as far as Hamilton was concerned, Mercedes weren't hiding anything in pre-season testing.
"The time you saw was the time we could do. I didn't feel when I got out that I could have gone faster.
"But of course there are all sort of things that come into play to bring the time down, such as fuel loads, engine modes, those kind of things. The great thing is no one really knows.
"It looks very positive for Ferrari, that's for sure, and I don't think that's a bluff."