The two horses fell over a hole in the San Siro racecourse in Milan within an hour of each other on Tuesday morning, La Stampa reported.
Angela Paula Terase, a 52-year-old amateur jockey and horse trainer, was warming up her horse when the animal tripped over the hole 15 meters from the finish line.
The fall unseated the rider, who fell to the ground. Paramedics arrived immediately and Terase was rushed to hospital, where she was put in an induced coma.
Police say the hole is around 20cm deep and 40cm wide, and suspect that it was made, either intentionally or by accident, some time on Monday.
“No hole should ever have been there,” Fabio De Pasquale, the public prosecutor who opened the case, told La Stampa.
It is thought that the hole was man-made, but police are not ruling out the possibility that it was made by the hoof of a galloping horse.
Police are also investigating why, following the first fall, the hole was not spotted or filled in by the racetrack maintenance team - which led to another serious incident shortly afterwards.
Just one hour later, Gregorio Arena, a professional 29-year-old jockey from Catania, was unseated as his horse, Pittura d'Arcadia, fell over the same hole.
Arena was also seriously injured in the fall and was rushed to hospital, where he underwent surgery for a subdural hematoma.
Following the second fall the track was closed and an investigation opened.
Both horses escaped uninjured.