A lack of engine power dogged Quartararo's title defence in 2022, with Yamaha forced to abandon its new engine prior to testing last year due to reliability issues and run a specification closer to the 2021 version.
Since then, Yamaha has been hard at work to develop a new, more powerful engine, bringing various versions to tests at Misano and Valencia last year prior to this weekend's pre-season outing in Malaysia.
Quartararo, who was fourth fastest on Saturday's rain-impacted second day of running, put Yamaha joint second on the speed trap figures at 335.4km/h, just 1km/h shy of Ducati.
The 2021 world champion admits now that Yamaha's bike improvement has been "much more than I thought" and believes the Japanese brand is now much closer to Ducati than it was last season.
"Well, in 2022 we had no [new] engine," Quartararo said. "That's something to say. Even when we started the first test, we knew the engine was going to be the same as the previous year.
"So, already today and before we started the test, we had the meeting with Yamaha and everything is looking much, much better than in 2022.
"So, it's not looking at all like last year. I was alone [when I set my best speed], so it's very promising.
"They are not giving up, so if they can find more and more it's going to be important.
"The top speed is good but our aero is much lighter, so this will be something important, to find more power to put more aero and downforce [on the bike].
"If it was further away, I would not be smiling. But of course, we made a massive improvement.
"From 2020 to 2022, the improvement was [small], and I think from 2022 to 2023 the improvement is much more than what I thought.
"So, Yamaha is working hard. It's not finished. In Portimao we will have more to test, maybe not on the engine, but a few parts that can make the bike a little faster and turn a little more."
Quartararo also revealed he does have more than one engine specification to test in Malaysia.
New on the Yamaha on Saturday was its attempt at what is now being dubbed in the paddock as the 'ground effect fairing', which features diffusers on the side of the lower bodywork.
Several manufacturers have been running with this aerodynamic configuration for some time now, which is aimed at aiding cornering.
Quartararo feels it's too early to say how much of a gain this development is on the Yamaha, noting: "There is a slight difference. Not a lot, but of course slightly slower because when you add some aero on the side of course it's a little bit slower.
"I didn't see it clearly [if it helped with turning], but of course tomorrow I want to try with new tyres to be 100% sure if it's better or not."
