The Mercedes-GP driver, who is fourth in the championship race, 44 points off leader Sebastian Vettel, said he owed his stunning performance in the final seconds of a rain-soaked qualifying session to the conditions.
And with rain forecast for Sunday in the Belgian Ardennes, Hamilton was quick to predict that his 31st career pole and fifth this year was no guarantee of winning the classic 44-lap race for a second time, having previously won it in 2010.
He is bidding to be the first driver this year to win two races in a row after becoming the first British driver since Damon Hill in 1995 to take four poles in succession.
Hamilton was outside the top 10 in both Friday practice sessions and only 12th in final practice on Saturday morning. He admitted that he could not have taken the pole in dry conditions.
"I would be guessing and my guess would be probably not," he said. "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 ... What a blessing, I feel so fortunate."
His conviction that Red Bull Racing is fundamentally faster than Mercedes only boosted his elation, particularly given his comfort in mixed conditions.
"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.
"I haven't had a good feeling all weekend. I'm not 100 per cent confident we have the fastest package in the dry, but in these conditions, it's more of a lottery and the driver can make more of a difference.
"Our radar has forecast it's going to rain tomorrow, so it's going to be a tricky race. Trying to choose the right tyre, tyre management and pit stops will be key tomorrow."
The pole spot changed hands five times in the closing stages of the session as Paul di Resta had briefly looked as if he would cause a surprise after going out on intermediates immediately at the start of the session, while the other nine drivers in the final session all mistakenly chose slick tyres.
But once on intermediates themselves, Di Resta's time came under threat, and Nico Rosberg, Hamilton's teammate beat the time in the last minute of the session.
But the German's effort was then beaten by Red Bull's Mark Webber, before his teammate Vettel went even quicker, before Hamilton stole the show.
But Vettel was not too disappointed to start in the second spot. The German is targeting a fourth successive drivers' championship.
"It was a tricky session for all of us. When I was on the last lap, I didn't know how the conditions were on the other side of the track," Vettel said.
"Lewis had a fantastic middle sector, I saw him catching up and thought, 'I could have gone quicker here and there'.
"It was a good result for myself. Everything is possible, we expect the rain to be as inconsistent as it was today."
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