Hill, 24, was breathing but unconscious when he was airlifted from Donald Racecourse to The Alfred hospital in Melbourne about 4pm.

A hospital spokeswoman said Hill's condition improved from critical to serious on Sunday night.

Hill was injured in the fourth race of the day, when he fell from The Muckle Toon, trained by Ciaron Maher.

Hill was in the middle of the pack coming around the final turn, when the three-year-old colt clipped heels with another horse.

Hill fell heavily, with The Muckle Toon landing on top of him.

Paramedics treated Hill for about 40 minutes before he was airlifted.

The rest Friday's meeting was abandoned at the request of the other jockeys.

Hill rode in the three previous races, claiming a second with Darren Weir-trained Miss Dubois.

Wimmera trainer Terry O'Sullivan had booked Hill to ride in the final race of the day.

"It is very distressing and sad and we just hope and pray he is okay," he said.

"Jack unfortunately clipped heels on the corner, the horse had a horrendous fall.

"I've seen a few fairly nasty falls, and thankfully it is uncommon."

A Racing Victoria spokesman said there would be an inquiry into the incident at a later date.

"The stewards run inquiries into any racing incidents," he said.

"In a scenario such as this, when a person is unable to participate, the inquiry has been adjourned and the date will be announced at some stage.

"Jack is in expert medical care at The Alfred and they are taking good care of him.

"From what I heard the horse didn't have any notable injuries."

Trainers Darren Weir and Lee Freedman and star jockey Tommy Berry were among many racing identities to express support for Hill via social media.