The double F1 world champion and veteran of 356 F1 races was also the fastest of all, in FP1 and FP2 as Lawrenec Stroll's team finally produced what seems to be a good car in the AMR23.
Alonso, now 41, joined Aston Martin from Renault-owned Alpine at the end of last season when five-times champion Sebastian Vettel retired.
"The whole weekend has been unreal for us," the Spaniard told reporters after a floodlit qualifying that saw Red Bull sweep the front row, with champion Max Verstappen on pole, and Ferrari fill the second row. It was like too good to be true every session and every performance of the car.
"Now there is not much more to downplay anything because this is qualifying, all in the same conditions and we are in the mix with Ferrari and Mercedes. So again, unreal, he added.
Alonso has not won a race for a decade, when he was at Ferrari, and his two titles came with Renault in 2005 and 2006. His last podium was a third place for Alpine in Qatar in 2021.
We have been competitive in all the sessions
In the team report the veteran added: "The whole weekend has been very good for us so far and we are very happy to be starting inside the top five for the first race.
"Last year the team found itself out of Q1 and now we are lining up fifth on the grid. Everyone in the team has done an amazing job and this is a strong baseline to build upon as we start the season. We shall see what it brings.
"But we have been competitive in all the sessions and we are starting close to the podium positions. If there is an opportunity there for the taking, then we will look to capitalise on it.
"It feels so strange for me to say that we’ll start P5 and maybe we are aiming for more Now we’re saying that we start P5 and maybe we fight with the Ferraris for the podium.
"Going for a podium in race one, it feels too good to be true," reckoned Alonso, who today will start his 357th Grand Prix, having made his F1 debut at the Australian Grand Prix on 5 March, 2001.