His former rival Lance Armstrong, who survived testicular cancer that spread to his lungs and brain, immediately tweeted his support.
At age 37, Basso wasn't a contender to win the race, as the Italian was in his heyday before he was banned for doping. But his experience and pedigree — Basso finished second at the 2005 Tour and third in 2004 — meant his withdrawal was keenly felt by his team leader Alberto Contador.
The 2007 and 2009 champion must now tackle the most arduous two weeks of the Tour, with decisive climbs in the Pyrenees and Alps, without the assistance and moral support of his veteran teammate and training partner.
On what is often an uneventful day of rest and relaxation when riders recharge their batteries before the high mountains, a visibly shaken Basso appeared with Contador at a news conference and announced that just two hours earlier, doctors diagnosed a tumour in his left testicle that had been painful since he crashed on Stage 5.
Contador put his arm around Basso and vowed, his voice cracking with emotion, to do his best to win the race to honour his teammate.
Basso said he has cancer.
"I have a small cancer in the left testicle," he said. "I have to stop and go back to Italy."
But his Tinkoff-Saxo team said more tests are needed to be certain the tumour is cancerous.
"Probabilities are very high," Pierre Orphanidis, a team spokesman, said in an Associated Press interview. "We still need the further analysis to be 100 per cent sure."
Tumors can be benign, meaning they're not cancerous and don't spread to other parts of the body, or malignant, which means they are cancerous and can spread.
Armstrong, who came back from cancer to win the Tour in seven victories later stripped from him for doping, tweeted: "Thinking about @ivanbasso and wishing him the very best as he embarks on his cancer journey. #IvanSTRONG!!"
He and Basso had memorable battles on the Tour's roads when both were in their prime, long before Armstrong eventually confessed to doping. Basso served a two-year ban for his involvement in a blood-doping ring.
In what he called "a moment of weakness," Basso said at the time that he "attempted doping" but never actually went through with it. His wins at the Giro, one of cycling's three biggest stage races along with the tours of France and Spain, came on either side of that suspension, in 2006 and 2010.
Now dedicated at this 102nd Tour to helping Contador win, Basso was in 158th place — out of 185 remaining competitors — and trailing race leader Chris Froome by more than 50 minutes after nine stages.
His team said Basso will have surgery to remove the tumour and that other treatment will depend on the findings of more checkups.
"It has been a blow to all of us," said Contador. "The entire team will give its best in order to get the yellow jersey and enjoy it in Paris with him."
Contador, who won his second Giro d'Italia this May, is attempting to become the first rider since Marco Pantani in 1998 to win that race and the Tour in the same year. But he heads into the Pyrenees already trailing Froome by 1 minute, 3 seconds.
The time gaps will force Froome's main rivals to attack him in the mountains. Unlike last year, when the-then defending champion had to pull out injured on Stage 5, he safely negotiated bone-breaking crashes, peloton-splitting winds and, on Stage 4, teeth-rattling cobblestones on this Tour's opening swing from Utrecht in the Netherlands, through Belgium and across northern France.
"If nobody attacks, we've won the race," the manager of Froome's Sky team, Dave Brailsford, said Monday.
Tuesday's 15-kilometre (9-mile) final ascent to the Stage 10 finish at La-Pierre-Saint-Martin is sufficiently long and arduous to provide the first acid test of which riders are genuine contenders for victory in Paris. The ski station perched high in the Pyrenees is known for its underground network of caves. The climb up there, with leg-burning 10-per cent gradients in parts, could swallow the podium ambitions of contenders who struggle.
It's the first big chance for lithe, lean climbers to shine. Froome, a human toothpick in Lycra, looks to be the strongest of them and seemed to actually be looking forward to the pain. His closest challenger, Tejay van Garderen of the BMC team, is 12 seconds back.
"This is the heart of the race," Froome said Monday. "All the action is going to be happening. We are going to see who has done their homework, who has got what in the mountains. This is where the real race for yellow truly starts. "
Jamey Keaten in Pau contributed to this report.