The MotoGP season kicks off in Qatar this weekend with fresh challenges and old rivalries as Valentino Rossi and Marc Marquez square up again for the first time since last year's toxic finale.

An aggrieved Rossi, now 37 and the oldest rider on the grid, decried a Spanish "stitch-up" in November after accusing Honda's Marquez of helping Jorge Lorenzo win his third MotoGP title for Yamaha.

Rossi, Lorenzo's teammate and title rival, finished fourth in that race in Valencia after being sent to the back of the grid for a clash with Marquez in the previous race in Malaysia.

Marquez, a double world champion and still only 23, has taken on the role of Rossi's arch-enemy after the Italian accused him of acting like Lorenzo's "bodyguard" and not attempting to pass.

Lorenzo is not far behind in the bad books after suggesting that the Italian, who boasts the longest winning career in Grand Prix history and is one of the all-time greats, will never win the 10th world championship he craves.

The bad blood between some of motorcycling's biggest names led the governing International Motorcycling Federation to intervene in ­October, accusing them of having "poisoned the atmosphere."

Rossi, who won under the Qatar floodlights in last year's opener, is not one to forget a grievance but is also eager to get back to racing.

"After what happened, there can never be any kind of personal relationship between us," he told Italy's Gazzetta dello Sport last week. "He [Marquez] really betrayed me. He arrived [in MotoGP] saying he was a fan of mine but it was all lies."

Rossi, who will be starting his 21st season in Grand Prix racing, hopes to be back on the podium at least on Sunday.

He singled out Lorenzo, who dominated preseason testing in Malaysia and Qatar with the fastest times, as the man to beat.

Marquez and his Honda teammate Dani Pedrosa have had more difficulties in coming to terms with electronics changes and a switch to ­Michelin tires but are also sure to be in the mix sooner or later.

Spaniard Maverick Vinales, a former Moto3 champion and still only 21, has attracted attention with strong testing performances with Suzuki.

The new season will have riders using the same control units and software provided by Magneti Marelli, which should level the playing field, while Michelin have replaced Bridgestone as official tire supplier after a seven-year absence.

The only rookie in the top category is Spain's 2014 Moto2 champion Tito Rabat. Former champion Nicky Hayden of the US has departed along with Italians Alex de Angelis and Marco Melandri.

The 18-race championship ends in Valencia again in November.