The US rider headed the time sheets on his KR211V with a best lap of one minute 56.905 seconds, beating Loris Capirossi's 2005 pole-setting time.

World champion Valentino Rossi (Yahama) was sixth fastest, behind fellow Italian Capirossi (Ducati) who won the opening race of the season in Jerez.

Britain's James Ellison was down in 17th on his Tech 3 Yamaha.

Roberts, who races for his illustrious father's team, said: "It's been a long time for the team and for me but it feels good to be back at the top.

"When you've got such a great engine and such great tyres then it comes easy to go fast."

Casey Stoner of Australia was second fastest behind Roberts with a time of 1:56.91 on his Honda.

His team-mate Marco Melandri of Italy registered the third-best time of 1.57:01.

Qatar MotoGP Thursday first practice times:

1. Kenny Roberts Jr. (US) Team Roberts, 1:56.90
2. Casey Stoner (Aus) Honda, 1:56.91
3. Marco Melandri (Ita) Honda, 1:57.01
4. Colin Edwards (US) Yamaha, 1:57.02
5. Loris Capirossi (Ita) Ducati, 1:57.03
6. Valentino Rossi (Ita) Yamaha, 1:57.14
7. Dani Pedrosa (Spa) Honda, 1:57.36
8. Toni Elias (Spa) Honda, 1:57.39
9. Nicky Hayden (US) Honda, 1:57.45
10. Sete Gibernau (Spa) Ducati, 1:57.56

17. James Ellison (GB) Yamaha 1:59.62