Team Sky's Chris Froome could possibly do so if he races and wins the 2018 Giro d'Italia in May, but that would be three within 12 months, not within one calendar year. Either way, no cyclist has ever won all three in such a short time frame.
"I would have liked, like anyone else, to have won all three Grand Tours in one year," Contador told Span's Marca newspaper.
"I've been able to win them, but in different seasons. If I had tried it when I was younger and when I had very powerful team around me, maybe I would have had more options."
Contador won more Grand Tours than any current rider has during his career. He took titles in all of cycling's three-week races, the Giro d'Italia, the Tour de France and the Vuelta a España.
The Spaniard from Madrid's outskirts won his home race three times, in 2008, 2012 and 2014. He won the Giro twice, 2008 and 2015, and the Tour two times, 2007 and 2009.
A positive test for clenbuterol saw him stripped of the 2010 Tour and 2011 Giro titles.
He recently retired with team Trek-Segafredo having begun his career with ONCE before racing with Discovery Channel, Astana and Saxo Bank/Tinkoff.
'El Pistolero' appeared to be firing the strongest between 2007 and 2009 when he raced with team Astana. However, he never tried racing all three Grand Tours in one season.
He came the closest to capturing all three within a 12-month span when he won the 2014 Vuelta and returned in 2015 to attempt the Giro/Tour double. He succeeded in the Giro and placed fifth in the Tour behind Chris Froome, saying the Giro had taken its toll.
Contador stands among the greats, a group of six cyclists who have won all three Grand Tours. Eddy Merckx remains the king of cycling with 11 titles. The Belgian won the Giro and the Tour five times each and the Vuelta once, the only time he raced it in 1973.
"I made the best decision," 34-year-old Contador said on his choice to retire.
"When I see a Grand Tour next year, maybe I'll get a little nostalgic. Recently I was at the presentation of the Giro d'Italia in Israel and seeing the first stage, the time trial, I thought, 'I could do well.' But it's time to leave space for the others.
"I could have continued, head and body, but it's true that now I see the Tour and the same passes... In a sense it no longer has the same charm as it did the first few times."
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