Rossi's Yamaha team-mate Jorge Lorenzo covered the 22-lap distance in 42m28.452s, seven seconds faster than Rossi when he won the Qatar Grand Prix 12 months ago.
Lorenzo's fastest lap, 1m54.927s set with three laps to go, was also three tenths faster than 2015's best.
The Spaniard had predicted over winter that laptimes this year would be half a second slower, due to the less advanced electronics in particular, and Rossi said the race pace was a genuine surprise.
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"It was a very fast race, faster than last year, which nobody expected," he said.
"Essentially the pace was a surprise, because it was a great pace.
"Michelin did a very good job, and also the electronics are good.
"We expected more difference, to struggle more.
"But already with three [pre-season] tests, all of the manufacturers improved a lot."
Rossi finished fourth, having spent the bulk of the race at the back of a chain of Lorenzo, Andrea Dovizioso and Marc Marquez.
While noting that he felt there was less overtaking, he said he didn't have enough pace to make a move and challenge for a podium.
"I was hoping for some more fights, but maybe it was better last year, more overtaking," he added.
"This year was more one behind the other.
"But, anyway, it's not so bad. It's better to arrive fourth, two seconds from first, than arrive second or third, 10 seconds behind.
"I was strong, but not enough to try one attack, for one sector, one braking.
"They beat us, we are behind, but anyway we are strong."
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