Tackling the Giro d'Italia-Tour de France double is more of a mental challenge than a physical one, former Grande Boucle winner Geraint Thomas told AFP.
Several riders who competed in May's Giro were also on the start line for the Tour, including eight of the top 10 finishers in Italy.
Of those, only Giro winner Jonas Vingegaard finished stage 14 in the top 10 at the Tour, and only two more were even in the top 20.
The likes of Australian Michael Storer, who was seventh at the Giro, was even outside the top 70, almost three hours behind leader Tadej Pogacar.
Several riders such as Storer and Dutchman Thymen Arensman, who finished fourth at the Giro, had said that they would be targeting stage victories rather than a high overall finish at the Tour.
But even then, none of those eight riders has yet featured prominently in any major breakaways, let alone come close to a stage victory.
For Thomas, the 2018 Tour champion who is now director of racing at Arensman's Netcompany Ineos team, the challenge of trying to secure high finishes in both Grand Tours is simply beyond most people.
"It is tough, especially when you try and ride GC (general classification) in both," he told AFP on Saturday.
"It's a mental strain as much as physical."
Arensman was one hour 11 minutes off Pogacar in 27th after 14 stages, while teammate Egan Bernal, a former winner of both Grand Tours, was 11th at almost 16 minutes, having finished 10th in Italy.
"I did the Giro-Tour once and it wasn't pretty, so I didn't necessarily enjoy the Tour after that," added Thomas about 2024, when he finished third at the Giro before struggling home in 42nd at the Tour.
"I was 38 and third in the Giro took a lot out of me and by the Tour I was just gassed.
"But again, for me it's a mindset thing as much as physical, so as long as the boys are up for it and keen to really go deep and suffer, then we'll see."
Both Arensman and Bernal had got themselves into an initial stage 14 breakaway of more than 30 riders, but they were not among the six who would survive until deep into the day before the overall contenders took over.
'IT'S TOUGH TO IMPROVE'
The last person to achieve the Giro-Tour double was Pogacar in 2024, but he was the first to do so in more than a quarter of a century.
Many others, including elite champions, have found out to their cost how hard it is to double up.
Four-time Tour winner Chris Froome attempted the Giro-Tour double in 2018.
He won the first but then finished third to his teammate Thomas at the Tour.
"Once I got into the last week of the Tour, I was running on fumes," he told AFP before the Tour began in Barcelona two weeks ago.
Two-time Tour winner Alberto Contador tried it in 2015, winning the Giro but then finishing fifth behind Froome in France.
He said that at the Tour he was "fresh mentally" but that his "body was still tired".
What this year has shown is that even those who competed fiercely at the Giro but are not targeting a top finish in France are struggling to make any impact at the Tour.
Vingegaard is the one rider who seems to be bucking that trend, to a certain extent.
Having won the Giro, he was second in the Tour ahead of Sunday's 15th stage.
His Visma-Lease-a-Bike sports director Marc Reef even expects the 29-year-old Dane's form to improve as the race enters the third week.
"We have trust in Jonas, that he really also grows into the race," Reef told AFP earlier this week.
"He feels good at this moment... the longer climbs are coming, so we're still confident."
Thomas, however, doubts how feasible it is for Vingegaard to finish strongly in the third week.
"I struggle to see how that happens to be honest," he said.
"I know he was in control in the Giro, but the Tour is the Tour and the start of this race has been so hard.
"Maybe he will come good, we'll see, time will tell, but for me it's tough to improve."