Ricciardo delivered a stunning lap of 1m 13.622s - only fractions shy of the fastest ever pole lap in the Principality - to secure the first P1 start of his career, with Mercedes' Nico Rosberg and Lewis Hamilton the only men able to get within half a second of the Australian.
"Amazing. That first lap? Mighty!" an elated Horner said in appraisal of Ricciardo's performance. "The first two laps in Q3 were just dynamite.
"He has been driving sensationally all weekend - just so clean, and the lap times just rolled together for him. It's a great moment for him with his first pole - and for us, with our first pole since [the Brazilian season finale in] 2013. A great way to start the weekend."
Having talked up his chances of pole all weekend, Ricciardo said he knew his lap would be difficult to beat as soon as he completed it.
"I knew we had a shot [of pole] - I had it in my mind the whole time, and we looked good on Thursday too," he explained.
"I put it together when I needed to. Qualifying built up and up, I found a rhythm, and that lap in Q3 was the one. I knew after I had done it that it would be hard to beat.
"This place more than ever is the one you want to do. I came in with a lot of confidence and belief that I could be in this position now, so I'm happy to fulfil that. It feels good!"
Asked about what impact the forecast rain could have on his race - and in particular the fact he will be the only front-running driver to start on the supersoft tyre, having set his best Q2 lap on the red-striped rubber - Ricciardo added: "Today we're sitting pretty, so hopefully it turns in our favour tomorrow.
"[The supersoft decision] seemed like a freebie for us - we did a good enough lap on the [ultrasofts], so we thought we'd see what happened - and it worked. Today the job was pole and I have done that; whatever happens tomorrow it's the same for everyone. I've done what I can so far."
WATCH: Ricciardo's inch-perfect Monaco pole lap
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