The hospital confirmed on Thursday that Hayden suffered "a serious polytrauma with subsequent serious cerebral damage".
"The clinical picture remains extremely serious and compared to the previous day there have been no changes".
Hayden competed in the Italian round of the Superbike World Championship at Imola last weekend.
Hayden was hit by a auto while cycling in Italy, and rushed to hospital with severe chest and head injuries.
The American was taken to hospital on Wednesday after being struck by a vehicle while training on his bicycle along Italy's Adriatic coast following a Superbike race at the Imola circuit at the weekend.
Doctors in Italy have confirmed former Moto GP world champion Nicky Hayden has suffered serious brain damage following a cycling accident.
The Maurizio Bufalini Hospital, where Hayden is being treated, said on Friday that his condition "remains unchanged" and "is extremely critical".
American motorcycle racer Nic.
Honda said he was taken initially by ambulance to a hospital near the coastal resort of Rimini before being transferred to Cesena.
"Nicky is one of the best friends I've ever had in the paddock", wrote the seven-time MotoGP champion on Instagram. Stay close as we will keep you updated on his condition as soon as new details emerge. In the MotoGP, he ran 218 Grand Prix races with three wins and 28 podium places.
The 35-year-old's fiancee and team members were at his bedside of the former MotoGP Champin who had won the Championship in 2006.
Wednesday's accident in Italy came just under a month after veteran Italian professional cyclist Michele Scarponi was killed after being hit by a van while out on a training ride on roads near his home.