Fernando Alonso says he is more worried by Ferrari's pace than the form of McLaren team-mate Lewis Hamilton.
Hamilton (left) and Alonso lead the championship so far this year
Four races into the season the Briton is top of the standings on 30 points, two ahead of world champion Alonso.
"I look at him like I look at Felipe Massa or Kimi Raikkonen, I must beat them all to be champion," he said.
"But he worries me least as he's my team-mate and we're here to help each other. I worry more about the pace of Ferrari than if he's two points ahead."
Hamilton's second-place finish behind Ferrari driver Massa in the Spanish Grand Prix saw the 22-year-old become Formula One's youngest-ever championship leader.
He's quite nationalistic so being beaten in Barcelona will hurt that little bit more
It was Hamilton's fourth podium finish in as many races, and also the first time since March 2005, when he was at Renault, that Alonso has trailed a team-mate in the standings.
Design chief Pat Symonds worked with Alonso at Renault and said the situation may be getting to the world champion more than he liked to admit.
"The thing he really hates is being beaten by his team-mate, and when it happened with us he could be come irrational - I suspect that will be the same again," he said.
"He's also quite nationalistic so being beaten in Barcelona will hurt that little bit more."
The next race on the calendar is Monaco, where Hamilton has raced three times in Formula Three and GP2, winning each time.
And McLaren chief executive Martin Whitmarsh said he felt both his team's drivers would be serious contenders for victory this time.
"I think Lewis has been gearing himself, as he tucks himself into bed at night, to the idea of winning in Monaco," he said.
"But the truth is that both of our guys are very quick there. I think they both fancy their chances and the car will suit there as well."
Britain's James Rossiter has joined the Super Aguri Formula One team as test driver.
The 23-year-old, part of Honda Racing's Young Driver Programme for two years, had been testing for Super Aguri's sister team Honda until February.
Rossiter will now join up with his new employers at this week's test at the Paul Ricard circuit in France.