Valentino Rossi evaluated his fitness to ride in this weekend's Aragon MotoGP round in a private test on a road bike at Misano on Monday.
The Italian rider had been expected to miss the Aragon race following a motocross crash at the end of last month, which left him with a broken right leg.
World Superbike rider Michael van der Mark has already been selected by Yamaha as a replacement for Rossi at the Spanish event.
But it emerged on Monday that Rossi had been at Misano to assess his fitness less than three weeks after his crash - well short of the 30-40 day recovery period advised by doctors.
A source told Autosport the Italian was able to complete a handful of laps before the test was interrupted by rain.
Rossi is understood to have felt pain in his right leg during the test, but could not determine from the limited running whether he would be fit to ride at Aragon.
Weather permitting, Rossi will attempt to complete further mileage on Tuesday.
Yamaha issued a statement on Monday evening confirming the test had taken place on a YZF-R1M and that Rossi will make a decisionon Wednesday as to whether he competes at the Spanish venue.
It read: "Following an enduro accident on August 31st, Valentino Rossi has been diligently working on his recovery from the double fracture in his right leg.
"Today [Monday] the nine-time world champion put his fitness level to the test and successfully completed a few laps around the Misano World Circuit Marco Simoncelli on a YZF-R1M, before rain disrupted the trial run.
"As today's riding session was cut short due to the change in weather, the evaluation of Rossi's physical condition was inconclusive.
"He will decide by the end of Wednesday, September 20th, whether he will attempt to take part in this weekend's Gran Premio de Aragon."
Yamaha boss Lin Jarvis previously suggested that Rossi would only attempt to race at Aragon if he felt he had the chance of fighting in the top five, but did not discount the prospect of the Italian making a comeback.
Rossi is fourth in the MotoGP riders' standings after he was forced to skip his home round at Misano, and is 42 points behind joint championship leaders Marc Marquez and Andrea Dovizioso.