The 53-year-old Zanardi, who had both his legs amputated after a motor racing accident almost 20 years ago, lost control of his handbike while competing in a relay race in Tuscany, Italy.
In the hand-written letter published by Italian newspaper Gazzetta dello Sport, Pope Francis praised Zanardi for living life to the fullest and for providing a “lesson in humanity.”
“Your story is an example of how to start again after a sudden halt. Through sport you have demonstrated how to live life as protagonists, making disability a lesson in humanity,” he wrote.
“Thank you for giving strength to those who had lost it. In this painful moment I am close to you, I pray for you and your family. May the Lord bless you and Our Lady keep you.”
Following the accident, Zanardi was airlifted to the Santa Maria alle Scotte hospital in Siena where he underwent three hours of emergency neurological surgery for facial head injuries.
The hospital said the Italian’s condition had remained “stationary” after his fifth night since the crash.
“The patient is always sedated, intubated and mechanically ventilated and the prognosis remains confidential,” read the statement.
Zanardi’s son, Niccolo, posted a picture on Instagram Wednesday of what appears to be him holding onto his father’s hand.
“I’m not letting go of this hand. Come on dad, even today a small step forward,” read the caption.
Livia Borghese and Paul Gittings contributed to this report.