The Briton had languished outside the top 10 in both Friday practice sessions, and despite overnight confidence was only 12th fastest on Saturday morning.
Amid the wild unpredictability of qualifying on a wet/dry track however he and Mercedes timed their run to perfection, with the Briton sealing his fourth straight pole as the chequered flag flew.
Asked if he could have achieved pole in the dry, Hamilton said: "I would be guessing and my guess would be probably not.
"Red Bull was looking particularly quick and also Ferrari was looking quite quick in the dry conditions, so [I'm] not necessarily sure we had the pace to be as fast today.
"I was so surprised when I crossed the line. When I started the lap I saw on the screen I was seventh or eighth and thought 'oh no', and it was raining more.
"I then went wide in Turn 1 and my dash display said I was three seconds down, then five seconds and six seconds, but I kept pushing, and could see I was catching Seb [Vettel].
"What a blessing, I feel so fortunate."
Hamilton said his conviction that Red Bull is fundamentally faster than Mercedes only boosted his elation, particularly given his comfort in mixed conditions that are likely to hit Sunday's race.
"I feel Red Bull is ahead of us a little bit in performance which is why today feels even better," he said.
"I think this weekend has shown that they have the pace, but I am hoping that whatever the conditions are, we can push.
"I feel quite comfortable in changing conditions and feel like I am able to find the limits when the conditions are on the edge."