Defending champion Fabian Cancellara has admitted an upset could be on the cards when he puts his time trial crown on the line at the road world championships on Wednesday. Cancellara, known as 'Spartacus' by his peers because of his unrelenting style in races against the clock and some of cycling's toughest one-day classics, is the owner of a record four world titles in the discipline. Usually considered unbeatable when on form, the Swiss star, however, approaches the 46.4 km race with question marks over his form. He was beaten by nearly a minute and a half by German rival Tony Martin at the recent Tour of Spain in a 10th stage time trial over 47 km in Salamanca. It was a significant defeat, and although Cancellara remains defiant it has left him pondering Martin as his possible successor. "He's the favourite," said Cancellara, who has won the title four of the past five years, during which he also took Olympic gold in Beijing. "But I'm not too worried. It's a one-day race. I can win it." Both Cancellara and Martin quit the Tour of Spain early, the Swiss leaving five days before the finish in Madrid and Martin leaving two days later. Despite Martin's evident form, Cancellara believes it could work in his favour. "It's a challenge for me when I see my rivals in such form," he added. Martin, who has won bronze in the event the past two years, looks the best challenger on paper. However, the flat, technical course may better suit a 'rouleur' -- a powerful rider who can reach speed on the flat and keep on enduring the pain for up to an hour. One such rider is Bert Grabsch, the German whose win in Varese, Italy in 2008 is the blip in Cancellara's winning streak. Three years ago, and fresh from winning the Olympic title, the Swiss opted not to defend his title. Britain's Bradley Wiggins and David Millar, the latter winning silver behind Cancellara in Geelong, Australia last year, are also among the contenders. Scotland's Millar refuses to rule out Cancellara, but said: "Obviously Cancellara is always good. But this course is one for the big (powerful) guys, like Grabsch and (Canada's) Svein Tuft."