He knew he could rely on his Ferrari's great start. And two of the four cars in front of him were the Mercedes of Nico Rosberg and Lewis Hamilton, which everybody expected to go backwards at a rate of knots in the race.
"We knew," Alonso said as he basked in the adoration of his home fans after the victory, "that today, if everything goes well and we do a fantastic race, we can win."
But this was not just about winning his home race, important though that was. Alonso was well aware that he badly needed to beat Red Bull's Sebastian Vettel, already 30 points ahead of him in the championship after just four races.
The early laps would be key.
"We knew we had the pace on the long runs and we wanted to have some clear air to exploit this potential in the car," Alonso said.
Alonso made the expected flying start, but on the run to the first corner he found himself boxed in behind Red Bull's Sebastian Vettel and Raikkonen.
"We knew that to win the race we needed to pass people at the start," Alonso said. "The start was very good but then it was very narrow and we didn't have the space to move a little bit.
"So, I wait for a better opportunity. It came straight after Turn One.
"I saw Kimi and Lewis running a little bit wide in Turn One so I changed trajectory and I had a clean exit from Turn Two. I passed Kimi and I said: 'Why not also Hamilton?' I had a little bit of Kers power-boost that I saved from the start for Turn Three, so I used that to pass Hamilton and I think that was a lot in the race."
Indeed it was. Now third, he trailed Rosberg and Vettel to the first pit stops, when he jumped the Red Bull by stopping a lap earlier.
Four laps later, Alonso was past Rosberg, by now in the expected tyre trouble. And that gave him the clear air he craved to unleash the performance of the Ferrari and the metronomic consistency for which he is by now famous.
By half distance, the destiny of the race was clear. On a four-stop strategy compared to Raikkonen's three, Alonso still had to do two stops remaining and Raikkonen just one, but he was already nearly a pit stop ahead.
When he made the first, on lap 36, Alonso came out only 1.8 seconds behind the Finn and on tyres that were 10 laps fresher. The race was effectively over.
He streaked into the distance and was nearly 10 seconds ahead by the time Raikkonen made his final stop nine laps later, and 30 in front when Raikkonen rejoined.
Even a puncture that forced Ferrari to bring forward Alonso's final pit stop could not disturb him. His 32nd career victory was in the bag, now fourth in the all-time winners' list, only Michael Schumacher, Alain Prost and Ayrton Senna ahead.
It was a win every bit as dominant as the one Alonso took in China two races ago, and it made one wonder what he might have achieved had it not been for the errors that cost him dear in Malaysia and Bahrain.
Spanish GP: Fernando Alonso celebrates with home fans
As it is, he is third, 17 points behind Vettel and 13 behind Raikkonen and it is very much game on.
Even at this early stage, it seems clear that the championship will be fought out between these three. Hamilton is hanging on in fourth place, but while his 39-point deficit to Vettel is not insurmountable in itself, with Mercedes' tyre problems it almost certainly is.
This is the third year out of his four with Ferrari in which Alonso has been in with a title shot and he admitted that this was probably his best chance of the lot "in terms of the performance of the car".
In 2010 and 2012 he missed out in the most agonising of fashions. Can he and Ferrari finally do it this year?
He might have taken two dominant wins in the first five races, and been competitive elsewhere, but Alonso - already on the back foot after the early-season errors - is under no illusions about the size of the task ahead.
"We should have won the championship, I believe, in 2010," he said. "We arrived in Abu Dhabi with seven points, or nine points ahead of Sebastian [it was 15]. We lost there but there is nothing we can change now.
"Last year maybe we didn't deserve it because we were one second off the pace but even with that we managed to fight until Brazil the final race.
"So, this year we have in our hands a package that maybe is not the fastest, still not the fastest, but we are working on that.
Alonso since his 2006 title
"But definitely on Sundays it's a very competitive team package, let's say.
"We are not the quickest over one lap, maybe we don't set the fastest lap time in the race. But we have fantastic strategy people, fantastic pit stops, the starts, tyre degradation. We have many ingredients to have a competitive car to fight for the championship.
"In the four years in Ferrari maybe it's true that this is the one we feel more confident with but we are not happy still with the performance of the car and we want to have a good and aggressive development programme for the next races."
Weighed against him are the unstoppable force of Vettel and Red Bull, who are still struggling to balance pace on Saturdays with tyre performance on Sundays, and the understated, icy consistency of Raikkonen and Lotus, F1's immovable object.
Each is formidable in their own way - especially Vettel and the might of Red Bull. But, even with their self-imposed handicap, Alonso and Ferrari look like they will take some stopping.
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