A late attack in Sunday’s ninth stage lifted the Tinkoff-Saxo rider to second in the overall standings, three seconds behind Colombian Nairo Quintana.
Quintana’s Movistar team mate Alejandro Valverde of Spain, who started the day in the red leader’s jersey, dropped to third, eight seconds off the pace.
“I am now able to say I will try to win the Vuelta, another thing is if I have the legs or not,” Spaniard Contador told a news conference on the rest day.
“Until yesterday, even up to the final kilometres, I did not see it like that and we have to wait and see how I am tomorrow,” added the double Tour de France champion.
“Since the start of the race I merely thought about letting the days go by.”
Contador said he was still having “minor” problems with his leg and that Quintana and Britain’s Chris Froome of Team Sky remained the favourites.
Froome lost ground on Sunday and is fifth, 28 seconds behind Quintana.
“Some have come here with more work under their belts than me and can aspire to greater things, like Froome and Quintana, while I am still a question mark,” Contador said.
“I’m getting better every day and that’s good for me but it’s also true I lack the confidence you get from having solid preparation behind you.”
At a separate news conference on Monday, Quintana said he had “improved a bit” in time trials and did not expect to lose much time to his main rivals.
“For the time being the legs are fine and I expect to wake up tomorrow with a good feeling and be on top form,” he added.
The three-week Tour of Spain concludes on Sept. 14 with a time trial in the north-western city of Santiago de Compostela.
(Reporting by Iain Rogers, editing by Tony Jimenez)