"(Being favourite) does make me feel tense, as always, but nobody can criticise what I've done this year," the Italian Team Astana rider, who won the Giro d'Italia in May, told Reuters.
"I think it's pretty much normal. I know I've got nothing to prove after winning the Giro but I still want to do as well here as I possibly can," added the 28-year-old, whose Grand Tour breakthrough came in 2010 when he won the Vuelta.
"It's important to do well here because the world championships follow the Vuelta (on home soil at the end of September) and a lot of people will be watching my performance here as a guide to how well I can ride there."
Race organisers have included a brutal 11 summit finishes in this year's edition with the penultimate 20th stage taking in the grueling climb of the Angliru, forcing Nibali to focus on reaching peak fitness in the third week of the tour.
However, the Italian was quick to point out that he would not be resting on his laurels in the early exchanges of the 21-stage race.
"The Vuelta has a tough start with a team time trial (on Saturday) which should benefit Astana as we have some strong specialists like Jakob Fuglsang and Jani Brajkovic, and we'll be aiming to gain a few seconds advantage," he said.
"But in a race with so many difficult moments, almost anything can happen. I won't say I'm here to win, I never say that in a race, but I'm certainly here to achieve a good overall classification."
A decision on whether he will go all out for the victory in Madrid on Sept. 15 or ease back to ensure he is in top form for the world championships, his final major target for 2013, will be made early in the third week of the Vuelta.
"If I feel in good enough shape to win and have an advantage of just a minute or two, then I will go for broke in a single stage somewhere," the Italian added.
"If on the other hand, I'm suffering so badly I'm barely able to keep up with the rest of the favourites... I'll do things differently."
However, should he grab the leader's red jersey as early as Saturday or after the first mountain top finish a day later, Nibali will fight to ensure he is still wearing it in Madrid.
"A leader's jersey is always to be defended," Nibali added.
"Even if it would be quite a long way from there to Madrid."
Nibali can expect a tough challenge from local duo Joaquin Rodriguez (Katusha) and Alejandro Valverde (Movistar) with Team Sky leader Sergio Henao of Colombia also regarded as one of the favourites.