Paolo Bettini, the world's leading classics specialist, heads the line-up in the hilly Clasica San Sebastian today.
The 2004 road race champion, who returned to racing after missing this year's Tour de France, said that Spain's top one-day race had to be respected.
"My condition is not yet at a high level, but I will do my best," he said.
The Italian won the 225-kilometre (141-mile) Clasica in 2003 and was second a year later. He said his main objective for the second half of the season was the World Championships in Salzberg, Austria, which get under way next month.
"Here, I really can't count myself amongst the big favourites, but my condition is definitely better than it was this time last year," Bettini said. "We'll just have to take Saturday's race as it comes."
This year's course will have the major first category climb of Jaizkebel, with its summit 32km from the finish on San Sebastian's elegant seafront, as the main challenge.
"Jaizkebel will be the decisive moment of the race," Bettini, a winner of nine major classics, said.
Contenders likely to test their form on the eight-kilometre climb include the ProTour leader Alejandro Valverde of Spain, the Australian Tour de France challenger Cadel Evans, and Kazakhstan's Alexandre Vinokourov.
The one-day specialists George Hincapie of the United States, the Italians Danilo Di Luca and Davide Rebellin and the Spaniard Juan Antonio Flecha will also be taking part.
"Rebellin could be a real factor, he has been showing strongly recently and he's won here before," Bettini said. "Valverde finished second in the World Championships after two months without racing, so you simply can never rule him out either."Reuse content