The two-time world champion lined up fifth on the grid as rain arrived in time for the race start, with the whole field shod with slick Pirellis.
Eventual race runner-up Alonso squeezed around the outside of fellow Spaniard and Ferrari driver Carlos Sainz through Turn 2 to cement his starting position before diving to the inside of the banked Turn 3 left-hander.
As Mercedes racer George Russell and Williams' Alex Albon opted for a more conventional dry high-line around the bowl, Alonso swept through the bottom of the curve to emerge in third place behind Lando Norris.
Alonso said the inspiration for his double overtake came from experimenting in Friday practice.
"It was FP1 or FP2, it was wet and in one of the out-laps I let a few cars go on the normal racing line and I found a lot of grip on the inside casually, by just letting people go," Alonso revealed.
"So, I kept it in my head, all the time, all the weekend, in case it was wet.
"I was ready to try again and, to be honest, when we were on the grid, and the red lights were on for the start of the race, there were a lot of drops in the visor.
"So, that's the worst feeling you can have because you are ready to start the race on a slick tyre and on your visor, you see that it's raining.
"So, you have no idea of the grip that you will find in Turn 1 braking, or Turn 3."
Alonso says he predicted everyone would take it easy on the drizzly first lap through the banked corner, especially since the top of it is bordered by more slippery advertising banners painted on the asphalt.
This enabled him to pull off what he thought was worthy of "overtake of the month".
Alonso continued: "So, I thought everyone would be a little bit cautious in Turn 3 with the grip level.
"Also, just outside Turn 3 there is paint, so you have to be a little bit cautious there. So, I thought about the inside line, it could work and yeah, we overtook those two cars.
"Definitely, that should be the overtake of the month, again. I hope."
