Marc Marquez became the youngest rider to win a MotoGP race after seeing off Repsol Honda team-mate Dani Pedrosa at the Grand Prix of the Americas.
The 20-year-old Spaniard, who started on pole, was involved in a thrilling tussle with countryman Pedrosa throughout the inaugural Texas race.
Pedrosa held off a late challenge from world champion Jorge Lorenzo to take second place, with the Yamaha rider finishing third.
Britain's Cal Crutchlow claimed fourth.
At the age of 20 years and 63 days, Marquez takes the record as the youngest rider to win a premier-class grand prix from American Freddie Spencer, who was 20 years and 196 days when he won the Belgian GP in 1982.
The win also means Marquez is top of the world championship standings after two races, level with Lorenzo on 41 points but ahead as the most recent grand prix winner.
"It is unbelievable," Marquez told BBC Sport. "A first victory in MotoGP is a dream come true. "I have to say thanks to all the people who helped me, especially the team because I could not do this without them."
Pedrosa swept past his team-mate seconds into the race but Marquez was never far behind, waiting to pounce on a chance to reclaim first place. That opportunity arrived with nine laps to go as Marquez slipped up the inside at Turn Seven.
Barely half a second separated the two as they duelled before Pedrosa's pace dropped with three laps to go.
"I did a bad lap time three laps from the end and I lost the race there," said Pedrosa. "But Marc had a good race, so congratulations to him."
After qualifying fourth, Crutchlow dropped behind Stefan Bradl early in the race but worked his way past the German on lap nine.
Bradl finished fifth ahead of Valentino Rossi in sixth, while Andrea Dovizioso took seventh place.
Alvaro Bautista was involved in a battle with Dovizioso for much of the race but could not find a way past the Italian and had to settle for eighth, with Nicky Hayden and Andrea Iannone completing the top 10.
Earlier, in the Moto2 race, Spaniard Nico Terol won ahead of compatriot Esteve Rabat, with Finland's Mika Kallio third.
British rider Scott Redding started on pole but dropped to fifth, while Spain's Pol Espargaro, winner of the season-opening race in Qatar last month, crashed out.
Spain's Alex Rins claimed his first Moto3 victory in a race that was decided by a five-lap finishing sprint following a red flag.
An earlier crash by Jasper Iwema delayed the session. When it was restarted, pole-sitter Rins held on to win ahead of Maverick Vinales and championship leader Luis Salom.
Grand Prix of the Americas results
1 Marc Marquez (Spa) Honda 43mins 42.123
2 Dani Pedrosa (Spa) Honda 43:43.657
3 Jorge Lorenzo (Spa) Yamaha 43:45.504
4 Cal Crutchlow (Gbr) Yamaha 43:48.739
5 Stefan Bradl (Ger) Honda 43:54.797
6 Valentino Rossi (Ita) Yamaha 43:58.738
7 Andrea Dovizioso (Ita) Ducati 44:04.497
8 Alvaro Bautista (Spa) Honda 44:04.977
9 Nicky Hayden (Usa) Ducati 44:15.896
10 Andrea Iannone (Ita) Ducati 44:24.235