With the Red Bull of Sergio Perez eliminated in Q1, Alonso was a strong contender to qualify on the front row for Sunday's Melbourne race alongside overwhelming favourite Max Verstappen, who duly took pole.
But, behind the world champion, Mercedes drivers George Russell and Lewis Hamilton proved the surprise package of qualifying by beating Alonso to start second and third respectively.
Despite being demoted back to fourth, Alonso felt Melbourne was Aston Martin's best qualifying performance yet this season because the gap to polesitter Verstappen was the smallest it has been so far.
"In terms of performance, arguably, this was the best qualifying for us of the three," Alonso said when quizzed by Autosport on his Albert Park performance.
"It's the closest we've been to pole position, so I feel fast and the car is easy to drive and I enjoy it."
In Bahrain, Alonso was fifth, over six tenths behind Verstappen while, in Saudi Arabia, he trailed Perez by 0.465 seconds. Then, in Australia, he reduced the gap to the Dutchman to 0.407s.
However, Melbourne's lap is over 11s faster than Jeddah so, in relative performance, the two-time world champion's claim does not quite hold up.
Alonso acknowledged it will be tough to work his way past Russell and Hamilton on Melbourne's street circuit, but is still aiming to get second.
"Let's see tomorrow what we can do - the Mercedes cars were very fast in Q3 and they did a better job, but let's see tomorrow if we can challenge them," the Spaniard said.
"They had very similar pace in Jeddah compared to me so, if they start in front, I think it's going to be very difficult to overtake them.
"Tomorrow I think the race is with Red Bull, the two Mercedes and probably the Ferraris as well - they're better here with these cooler conditions, so it's going to be an interesting race."
