McLaren drivers Jenson Button and Lewis Hamilton reckon Lotus emerged from Thursday practice in Monaco in the best shape.
Although Kimi Raikkonen lost the morning with a steering issue and managed little dry running in the afternoon, his teammate Romain Grosjean's second places in both practice sessions left rivals impressed with Lotus's pace.
"The cars that looked very fast for me were the Lotus," said Button. "Their runs whenever they are out look very competitive, so I'd say the Lotus is the car to beat so far this weekend."
Hamilton agreed that Lotus was a major threat, although he felt several teams had a chance to be near the front.
"The Lotus is massively quick," he said. "I'm not really quite sure what Red Bull is up to, but I have no doubts they'll be quick. The Ferrari is very quick.
"The same teams are quick this weekend. I think everyone's got the chance to fight for the front end, and it's all about making sure you get out there at the right time, you get the temperature into your tires and your brakes, and you nail it on the lap that you get."
Button managed to set the day's quickest time before the rain arrived in the afternoon, but did not think his pace was particularly significant.
"From P1 to P2 the car has improved, as there was something that I didn't like in P1, so we've tried something different and it's worked," said Button. "So I'm happy with the way the car feels, but we are still not there yet. We were quickest, but we were one of the only cars to do a timed lap on the super-soft.
"I don't think it's massively quick, but the car feels OK and I think there's a lot of room for improvement. We know the direction that we can take."
With only a handful of laps completed by anyone on super-soft tires on Thursday, and rain predicted for Saturday, drivers face the prospect of going into Sunday's race with no experience of the super-softs over a longer stint.
"The important thing on that tyre is to try to get a long run with it, which I don't think anyone did on high fuel and we don't really know what's going to happen," Button said.
Asked for his view on the tire situation, Hamilton added: "Everyone's in the same boat, so it'll be fun, that's for sure."