Zanardi, 53, who had both legs amputated after a car accident nearly 20 years ago, lost control of his motorbike while competing in the “Obiettivo tricolore” relay race in Tuscany, Italy on Friday.
According to many reports, he crossed the white line on the path of another track, which he could not avoid.
Following the accident, the Italian was airlifted to Santa Maria alle Scott Hospital in Siena, where he underwent three hours of emergency neurological surgery for head injuries.
A hospital statement said Zanardi was in “stable” condition after intensive care.
“It is induced and supported by artificial ventilation, while the neurological picture remains serious,” read the medical report.
Zanardi’s misfortune was recorded in the headlines of all major Italian sports newspapers as the news spread about the seriousness of his accident.
“No Alex, no!” Corrierre dello Sport said, while Italian Prime Minister Giusseppe Conte wrote on Twitter: “Come to Alex # Zanardi, don’t give up. All of Italy is fighting with you.”
Zanardi competed for Jordan, Minardi and Lotus in F1 before a successful transition to CART races in the US, where he was champion of the series in 1997 and 1998.
He returned to F1 with Williams for one season in 1999 before returning to the CART series.
In this formula he suffered the famous 320kmh accident in Lausitzring, Germany in September 2001, escaping with his life, but lost both his legs.
In an incredible show of determination and spirit, Zanardi adapted to his prosthetic legs and returned to compete in the 2003 European Touring Car Championship, driving for BMW.
But it was for his accomplishments in a handbike that the man from Bologna became known to a global audience, winning many medals at the 2012 and 2016 Paralympic and World Championships.
He was training for this year’s Paralympic Tokyo, hoping to add to his impressive number of gold medals until it was postponed.
Zanardi has also competed in exhaustive Ironman triathlon, setting a new world better for an Paralympic athlete in a race in Italy last September, and also continued to compete with specially adapted BMWs at various endurance racing events.
Messages of support have flooded the world of sports, with former F1 world champion and Indy Car legend Mario Andretti writing: “I’m so worried and scared about Alex Zanardi holding my breath. I am a fan of him. I am his friend.
“Please do what I do and pray, pray for this wonderful man,” he wrote on Twitter.
“Alex is one of the most inspiring people in life and, as we all know, a fighter in and out. Stay strong and Forza Alex,” the former Williams team posted on Twitter.
“Fight as you know how to do it, Alex. You’re a great man, courage,” said Ferrari star Charles Leclerc in Italian.