Jenson Button and his McLaren team-mate Lewis Hamilton warned on Thursday that the fight to win this weekend's Monaco Grand Prix is wide open and that this season's dark horses Lotus are a major threat.
Button topped the times overall after Thursday's two free practice sessions ahead of Lotus's leading driver Romain Grosjean and said afterwards that he was impressed with their speed.
"The cars that looked very fast for me were the Lotus cars. Their runs, whenever they are out, look very competitive, so I'd say the Lotus is the car to beat so far this weekend," said Button.
Hamilton agreed with Button and added: "The Lotus is massively quick, but I am 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 tyres and your brakes - and you nail it on the lap that you get."
Lotus' Kimi Raikkonen may have lost the morning with a steering issue and managed little dry running in the afternoon, but Grosjean's second places in both sessions left everyone impressed.
Button set his fastest time of the day before 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 tyres on Thursday, and rain forecast for Saturday, the drivers face the prospect of going into Sunday's race with no experience of the super soft tyres 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," said Button.
Hamilton said: "Everyone's in the same boat, so it'll be fun, that's for sure. Let's see."