Australian Jack Miller was fastest at a chaotic practice session for the season-opening Portuguese MotoGP on Friday as Spanish veteran Pol Espargaro was hospitalised after a violent crash which resulted in "severe spinal trauma."

Miller clocked a best time of 1min 37.709sec on a KTM, setting a new lap record at the Portimao circuit.

Maverick Vinales, of Aprilia, and reigning world champion Francesco Bagnaia, on a Ducati, filled out the top three.

However, the high-speed crash suffered by 31-year-old Espargaro on his KTM Tech3 GASGAS bike caused a long interruption to the session which eventually finished around two hours late.

The Spaniard, who moved to his new team after two disappointing seasons at Honda, lost control of his machine on Turn 10 in the second session and was catapulted off the track.

Espargaro, who rolled into a wall, was treated trackside for around half an hour before being airlifted to hospital by helicopter to Faro for treatment to a back and neck injury.

"He suffered a dorsal and thoracic trauma," said a statement from championship promoters Dorna.

Espargaro was "conscious" at the time "and was transferred to Faro hospital for further examinations."

Later Friday, MotoGP Medical Director Angel Charte said the veteran rider had "suffered severe spinal trauma and a contusion to his lungs."

He added: "Although I think he will recover well, we need to conduct the appropriate tests to determine the extent of his injuries."

"He is conscious, alert, and responding well. He is slightly sedated due to the painkillers we have administered. He can move his feet, legs, and arms perfectly well, so there is no reason to fear any permanent spinal injury."

A crash in the dying minutes of the session for six-time MotoGP world champion Marc Marquez on a factory Honda meant the Spaniard, who has undergone four surgeries in recent seasons, was out of the Top 10.

He will have to fight through the first qualifying session for a chance of a spot in the second run on Saturday.

Earlier, the session had also been delayed by technical issues which affected the timing mechanism.

The top 10 in practice on Friday wax completed by Luca Marini, Jorge Martin, Fabio Quartararo, Johann Zarco, Marco Bezzecchi, Aleix Espargaro and Enea Bastiaini.

As well as qualifying on Saturday, the championship will stage a sprint race for the first time after the format was introduced in Formula One last season.