Friday's opening practice session ahead of this weekend's Shanghai MotoGP was marred by a big accident for joint series leader Jorge Lorenzo.
The Fiat Yamaha rider, who won his first MotoGP race at the last round in Portugal, suffered a chipped bone in his left ankle and severe pain in his right ankle after coming off his bike.
The Spaniard did not return for the afternoon session and is now a major doubt to complete the remainder of the weekend.
Circuit doctor Claudio Costa said: "Detailed examination has revealed that Jorge has re-fractured the top of the malleolo bone in the middle of the left ankle.
"Furthermore, the MRI scan showed that there are severe impact oedemas to the right heel, the astragalo bone and the tibia, but fortunately no signs of further fractures.
"The diagnosis shows the violence of the trauma he suffered to both his feet in the crash and Jorge now faces a very difficult task to participate in tomorrow's qualifying and Sunday's race."
The rookie rider's accident could benefit compatriot and arch-rival Dani Pedrosa (Repsol Honda), who currently shares the lead of the championship with Lorenzo on 61 points and came third in each of today's practice sessions.
Those sessions were split between two former champions, with Italy's Valentino Rossi fastest in the morning and current title-holder Casey Stoner (Ducati Marlboro) taking the honours in the afternoon.
Rossi posted the overall fastest time of the day on the Fiat Yamaha in one minute 59.906 seconds while Stoner's session two-leading time of 2min 0.163secs was good enough for second overall ahead of Pedrosa (2:00.361).
Tech 3 Yamaha's James Toseland was 13th fastest on his first visit to Shanghai, and was happy with his progress on his track debut.
Toseland said on his official website: "The layout is possibly the most incredible I've ever seen and the track is really grippy.
"We've had two positive practice sessions and I'm confident that we can improve further tomorrow to enable me to demonstrate my full potential on race day."
Britain's Bradley Smith (Polaris World Aprilla) impressed in practice for the 125cc class, finishing second to Germany's Stefan Bradl by just 0.057secs.
Daniel Webb (Degraaf Grand Prix) finished the session in ninth place while Scott Redding (Blusens Aprilia Junior) was down in 20th.
Ireland's Eugene Laverty was 18th on the Blusens Aprilia in the 250cc session, which was topped by Alvaro Bautista.
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