Spain's Maverick Vinales, who won the season-opener in Qatar, continued his blistering form on Friday by setting the fastest time in practice for Sunday's Grand Prix of Argentina as his rivals struggled.
The 22-year-old clocked a best time of 1min 39.377sec after two sessions on the hot and dusty Termas de Rio Hondo circuit with defending world champion Marc Marquez second fastest on a Honda, 0.301sec behind.
Unheralded Czech rider Karel Abraham, on a Ducati, was third fastest at 0.403sec off the lead.
"I feel very positive. I feel great on the bike and physically I'm really fit," said Vinales.
"Step by step we are improving and we still have some work to do for the race, but I think we have the speed, so that's really important."
Marquez, the 2013, 2014 and 2016 world champion, recovered well after managing a mediocre 11th best in the morning run which saw the Spaniard suffer a fall.
"As always happens here in Argentina, today the track was quite slippery in the beginning, and I crashed this morning because I was pushing a little too hard and went a bit off line," he said.
"Anyway, after that we worked really hard and found a promising setup. We're still missing something, and tomorrow we'll try to improve in the second sector, especially the acceleration at turns 3 and 4."
However, it was a miserable two sessions for nine-time world champion Valentino Rossi, the teammate of Vinales, who was down in 16th in both sessions and ended the day 1.054sec behind his Spanish Yamaha partner.
"We have a lot of problems to enter the corners fast with this bike and I don't feel comfortable," admitted Rossi, the winner at the track in 2015 and second 12 months ago.
Former champion Jorge Lorenzo, whose decision to join Ducati created the Yamaha vacancy for 22-year-old Vinales, fared even worse, ending the day in 18th place, 1.122sec off the pace.
Rossi was third in the Qatar Grand Prix two weeks ago but Lorenzo, the 2010, 2012 and 2015 world champion, was a disappointing 11th in a race where his Ducati factory teammate Andrea Dovizioso was runner-up.
MotoGP practice times:
1. Maverick Vinales (ESP/Yamaha) 1:39.377, 2. Marc Marquez (ESP/Honda) at 0.301, 3. Karel Abraham (CZE/Ducati) 0.403, 4. Alvaro Bautista (ESP/Ducati Aspar) 0.445, 5. Danilo Petrucci (ITA/Ducati Pramac) 0.483, 6. Loris Baz (FRA/Ducati Avintia) 0.643, 7. Cal Crutchlow (GBR/Honda LCR) 0.647, 8. Jonas Folger (GER/Yamaha Tech3) 0.680, 9. Aleix Espargaro (ESP/Aprilla) 0.698, 10. Andrea Iannone (ITA/Suzuki) 0.773
16. Valentino Rossi (ITA/Yamaha) 1.054, 18. Jorge Lorenzo (ESP/Ducati) 1.122
1. Franco Morbidelli (ITA/Kalex) 1:44.188, 2. Marcel Schrotter (GER/Suter) at 0.149, 3. Takaaki Nakagami (JPN/Kalex) 0.226
1. Joan Mir (ESP/Honda) 1:50.248, 2. Romano Fenati (ITA/Honda) at 0.564, 3. Nicolo Bulega (ITA/KTM) 0.683