SEBASTIAN VETTEL has fired a warning to Lewis Hamilton and Jenson Button — insisting he spent the winter becoming an even better driver. The German is chasing his third consecutive Formula One world crown after claiming the 2010 and 2011 titles in his Red Bull. The 24-year-old, who won 11 of the 19 races last season, said: "The idea was to get away in the winter, take time to reflect and hopefully come back stronger, more prepared and more organised. "We have sat down as a team and talked through last year and all the things we can improve on. "Looking from outside it looked almost perfect but there were quite a lot of things on the list." Vettel was the third fastest on the final day of testing in Jerez, Spain yesterday. Fernando Alonso clocked the fastest time in his Ferrari. The Sun .