Rossi has built up momentum as the season has gone on

Defending champion Rossi, who is second in the standings, set a time of one minute 36.200 seconds, ahead of team-mate Colin Edwards.

Championship leader Nicky Hayden qualified in third place.

With two races of the season left, Rossi trails Hayden by 12 points, and is his only serious title rival.

Estoril is one of Rossi's favourite circuits, and is not one where Hayden has traditionally enjoyed good fortune.

Rossi has won there four times and never missed the podium.

Hayden has not even made the top six in his last two visits, and has finished outside the top three in his last four races.

Rossi said: "We did some great work with Michelin for tyres and three very good laps.

"But the last one was one where I kind of left my brain behind and just went for it. Going on the limit like that is a lot of fun.

"I'm very happy for Colin to be second - maybe he can help me, because I think Nicky (Hayden) will be very fast tomorrow."

Portuguese MotoGP qualifying times (Sat):

1 V Rossi (It) Yamaha 1 min 36.200
2 C Edwards (USA) Yamaha 1:36.478
3 N Hayden (USA) Honda 1:36.549
4 D Pedrosa (Sp) Honda 1:36.569
5 C Stoner (Aus) Honda 1:36.702
6 J Hopkins (USA) Suzuki 1:36.790
7 S Nakano (Jpn) Kawasaki 1:36.790
8 S Gibernau (Sp) Ducati 1:36.940
9 C Checa (Sp) Yamaha 1:37.107
10 L Capirossi (It) Ducati 1:37.182