Sergio Perez says he doesn't understand how his former team McLaren has struggled so badly after a promising start to the season.

Kevin Magnussen finished second and Jenson Button third (after Daniel Ricciardo's exclusion) in the season-opening Australian Grand Prix, but it has slowly fallen away since then and failed to score in each of the last two races. Perez believes Force India is fighting with Williams behind the leading trio of Mercedes, Red Bull and Ferrari, and is surprised by McLaren's plight.

"I think we're not far off Williams," Perez said. "I think McLaren - for some reason - have struggled quite a lot after the first race, which I struggle to understand why. They looked very competitive in the first race but for some reason they haven't been any more competitive.

"In Bahrain they were a bit more competitive but other than that they're not at the moment. But we know they are a good team so hopefully they don't come back!"

However, Perez said being ahead of McLaren is not changing the Force India mindset when it comes to targets for this season.

"We want to score as many points as possible. We cannot leave good opportunities; for example in Malaysia we had a really good opportunity to get in to the top six or seven and we had an issue with the software of the car. And in Australia we had another problem with the software, so it's very important that we do our best. Those problems cannot happen to us again and we need to score as many points as we possibly can in the next couple of races."

While he expects the field to settle down at this stage of the season, Perez still believes there will be big changes in competitiveness, particularly at the front. Asked if it would be impossible for Mercedes to be caught if it wins in Barcelona, he replied: "No, I don't think so.

"We have seen in the past that the Formula One season is so long; four races is nothing. They are quite far ahead and I believe they have a big gap to anyone else but I think Ferrari and Red Bull are going to close the gap quite a lot.

"I believe Red Bull has the best car. In my opinion they have got the best car on the grid."