London, Mar 8 : Red Bull chief Christian Horner has said former champion Jenson Button is the biggest challenge for his team this season, but cautioned rivals that there is more to come from defending champion Sebastian Vettel.

Horner believes McLaren's Lewis Hamilton, Ferrari's Fernando Alonso and Lotus' Kimi Raikkonen are also contenders this year. Each of them is a former champion except Raikkonen, who has finished runners up twice. "We are yet to see the best from him.

He will evolve and get stronger," The BBC quoted Horner, as saying. Last season, Button was placed a distant second and trailed by 122 points from Vettel, but Horner thinks it is going to be a much tighter title race this season. "Seb had an unbelievable year and in the end dominated significantly," he said. "But there are some big opponents out there, with six world champions on the grid.

Lewis is going to be pushing hard this year, Jenson had a great year last year, you can''t rule out Fernando because he is a world-class driver and it will be interesting to see how Kimi fares on his return," he said. When asked to pick out the strongest rival to Vettel and his teammate Mark Webber, Horner responded: "Probably Jenson." (ANI)