Alex Marquez, who had led by one point entering the weekend, battled for the lead throughout as the brothers pulled clear of Franco Morbidelli in third.

Defending world champion Jorge Martin, returning from injury, finished 16th in his first race of the season.

Over the first three weekends of the season, Marc Marquez, in his first campaign on a factory Ducati, was all-conquering until he fell while leading in the last race, the Grand Prix of the Americas.

On Saturday, he resumed his domination, taking his fourth straight pole positon and then his fourth straight sprint success.

"It was an unexpected performance because normally I have problems in Qatar," he said. "I was feeling in a very good way. I was super concentrated and controlling the gap."

Alex briefly overtook on his Gresini Ducati during the first lap but had to settle for second, allowing the lead to grow to 1.577 seconds in the closing stages.

He has finished runner-up in all four sprints and all three main races this season.

"For sure I tried," he said. "I also wanted to make the gap with the riders behind. Marc was better me."

Italian Morbidelli was 3.988sec adrift on a VR46 Ducati.

Spaniard Fermin Aldeguer, who is making a habit of finishing fast on his Gresini, overtook Frenchman Fabio Quartararo on the final lap to take fourth.

Francesco Bagnaia, the winner in Austin, started 11th on the grid but climbed to eighth on the second factory Ducati to claim two points as he remained third in the standings.

Martin, who missed the first three races after two bone-breaking pre-season crashes, started 14th, briefly climbed into the points but then subsided to 16th on his Aprilia.