Jenson Button says he is fearful that Red Bull Racing could stage a comeback now it has a revised diffuser arrangement, despite having won four of the first five races of the season.
On the back of a dominant start to the campaign, Button expects things to get much harder from now on – with Red Bull singled out as the biggest threat.
"It is getting harder and harder," said Button about his championship prospects. "In the last two races you would say that the Red Bull was as quick as us. If you look at the lap times, which I do at every race, the consistency of Mark (Webber) at the last race and Sebastian (Vettel) when he was in clear air in Bahrain, their pace is very good.
"But they haven't put the weekend together and we have – as a team, we've done a better job. When they do actually get everything together they're going to be quick, and I think here in Monaco they are going to be quick.
"I think McLaren has a good car mechanically, so they should be competitive around here. So yeah, it's going to be a tough weekend. But I am looking forward to it."
And although Vettel has had the best results for Red Bull so far, Button says he is more worried about the German's teammate Webber in Monaco this weekend.
"I think Webber is quick here," he said. "I've gone over all the previous years of team-mates against teammates and how well they've gone around here, and I'd say Webber is obviously a threat. Sebastian is in a strong car and Rubens is obviously in a strong car, so he is going to be a threat.
"As I said before I don't think you can count out McLaren, I think this is a good circuit for them. And strangely enough, KERS does make quite a bit of difference around here – I think it's three-tenths a lap, so it's quite a bit."
And despite his comfortable lead in the world championship standings, Button is refusing to get too carried away with his title prospects.
"We don't know what it going to happen for the other 12 races. Winning four out of five is a great stat and hopefully it continues, but I'm not going to let it get to me. Every race I finish, and obviously winning the last couple of races, it's a great feeling when you win a race, but as soon as it is over and you have enjoyed the moment, you are already thinking about the next one. So in a weird way it is quite stressful, but exciting stressful."