After George Russell and Lewis Hamilton grabbed second and third on the grid for the Australian Grand Prix, both drivers said the result was far beyond expectations after its difficult start to the season.
But Alonso, who has qualified fourth in Melbourne, sees things very differently and believes that Mercedes has a much better package in the W14 than it has been making out.
Asked whether he was surprised to see Mercedes beat him in taking the fight to pole position man Max Verstappen, the Spaniard said: "No, in Jeddah I think they were within one tenth of the pace on Sunday to us and here they were P2 yesterday with Lewis in FP1.
"If you read their comments, for sure it seems they have a car that is out of Q3. But I don't think it's that bad.
"It's not as good as Red Bull - no one has a car close to Red Bull - but they're getting better and they will be a contender for wins soon. They did it last year and they won a race after a very bad start, so I think this year they're showing the potential they have."
Alonso is not the only one to say that Mercedes is stronger in reality than the team has been portraying in public.
Ferrari's Carlos Sainz told Sky: "I think that they are a lot faster than what the people think. I think especially also in the race, they had a pace very close to Fernando in Jeddah. George and Lewis, they were flat out and we couldn't keep up with them.
"So that car is quick. It is clearly not as quick as the Red Bull is, no one is, but if they put qualifying together, they can actually be a very strong package too."
Alonso said that he hoped Mercedes could jump Verstappen at the start of the race on Sunday to prevent the grand prix being too boring.
"I think there are many unknowns into the race, after missing FP2 [running]," said the Aston Martin driver. "And also the temperature, I think it's going to be very different tomorrow, and it is at least supposed to be sunny.
"I think in terms of strategy and race preparation, I think no one is fully ready yet, so I think tomorrow is a race that we will discover who has better pace and better management and things like that.
"Let's see, at least, if the Mercedes they can make a good start and stop Max somehow. If not, there's going to be a boring race for him."
Additional reporting by Filip Cleeren
