Three-time Tour de France winner Chris Froome has named Australian Richie Porte among his biggest threats for a fourth overall victory in the grand tour, starting on July 1.
Colombia's Nairo Quintana, runner-up to Briton Froome in 2013 and 2015 and third last year, made an audacious bid for a rare Giro d'Italia-Tour double this season, but lost out to Tom Dumoulin in Italy.
And Team Sky's Froome believes Movistar's Quintana may have over-exerted himself in the process.
"My biggest threats come from guys who did not do the Giro -- Richie Porte, (two-time winner) Alberto Contador and (last year's runner-up) Romain Bardet," defending champion Froome told Reuters in a telephone interview on Wednesday.
"I think it is going to be tough for Nairo to do the Giro and the Tour."
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Quintana looked nothing like the ruthless climber he built his reputation on in Italy, failing to make Dutchman Dumoulin, renowned more for his time-trial speed, crack in the mountains.
Although he did not see much of the race because he was at a training camp in Tenerife "where there was no TV", Froome said Quintana could have been cycling within himself in Italy.
"He may then be doing better on the Tour because he is a rider who is better in the second than the first grand tour," 32-year-old Froome said.
Froome, however, tipped his former teammate Porte as a major contender.
"Richie has been amazing this season, winning the Tour Down Under and recently the Tour of Romandie," Froome said.
"I think this year's Tour suits Richie really well."
Porte, 32, was Froome's lieutenant at Sky from 2012-15 before leaving for American outfit BMC to become a team leader.
"He's definitely got a chance to be up there for the win," Froome said.
"This season he has been showing what he is capable of. I always believe in Richie."
Porte, however, has a tendency to suffer from damaging bad days that can kill off his chances in the grand tours.
