Nimes - Alberto Contador said on Friday he knew the time had come to retire during last month's Tour de France, where he was unable to keep up with the contenders.
The 34-year-old Spaniard begins his final Grand Tour on Saturday as the Vuelta a Espana begins in Nimes, France with a time-trial.
He had announced 10 days ago that he would retire at the end of this season following a glorious career in which he won the Tour twice, the Vuelta three times and the Giro d'Italia twice as well.
But in July he laboured to a ninth-placed finish at the Tour, his worst placing since coming 31st on his debut in 2005.
"It's a decision that I've thought long and hard about," said Contador of his retirement.
"I knew the Tour de France would give me my answer.
"I arrived at the Tour in great shape with the ideal weight."
Until then he had been in talks with his Trek team over a contract extension beyond the end of this season.
"The decision (to retire) had been 70 percent taken but there was a moment where I saw things clearly, after the Tour's ninth stage.
"I had to contend with things that weren't in my plans, such as crashes, and that's the day I got my definitive answer."
Contador has raced the Vuelta four times and last year, when he finished fourth, was the first time he'd failed to win it.
This year, though, his ambitions are limited.
"What's clear is that in every race I want to give it everything," he said.
"I've always been super professional and I'll remain that right to my last active day in the sport.
"This Vuelta is special, it will be three dream weeks, I'm lucky to have this opportunity."
Four-time Tour champion Chris Froome is bidding to achieve a rare Tour-Vuelta double, in so doing earning his first win in Spain's three-week Tour having finished second three times, including behind Contador in 2014.
And Contador, known as the sharp-shooter (El Pistolero), thinks his British rival will finally land the Vuelta.
"I think the rider who's prepared best for this Vuelta is (2010 champion) Vincenzo Nibali. He rode the Giro and then concentrated his preparations (on the Vuelta).
"There's also all the Tour riders like (Fabio) Aru (the 2015 champion), (Romain) Bardet or (Simon) Yates.
"But the favourite is Froome, he's really strong, especially in time-trials and thanks to his team-mates, who could be leaders in any other team."