MOTEGI, Japan (Reuters) - Honda rider Marc Marquez secured his third MotoGP championship in four years on Sunday by winning the Japanese Grand Prix after his nearest rivals Valentino Rossi and Jorge Lorenzo both crashed out.

The 23-year-old Spaniard had only an outside chance to seal the title but his win at Motegi gave him an unassailable 77-point lead over Italian Rossi, who started on pole but slid into the gravel early.

Already the youngest rider to win a MotoGP race and the title in his maiden season in 2013, Marquez's triumph in Japan made him the youngest to seal three championships.

He finished nearly three seconds ahead of Ducati rider Andrea Dovizioso, with Suzuki's Maverick Vinales third.

With Rossi, the seven-time premier class champion, quickly out of contention, Lorenzo needed to finish fourth or better to keep his title defence alive.

The Spaniard, who suffered a terrifying crash in practice on Saturday but returned to the track to qualify third on the grid, battled to keep within touch of Marquez.

But Lorenzo came under pressure from a hard-charging Dovizioso and his race ended in the gravel with five laps to go after a front end skid.

The championship, now a victory procession for Marquez, heads to Phillip Island, Australia next week.