Knox secured his playoff place by holing another huge putt at the 18th in his regulation round, with Fox missing a 10-foot putt that would have given him the title.
Knox and Fox headed down the 18th once more, both had great drives and found the green easily.
Somehow, he sunk it again, roaring with delight, sending the crowd into raptures, and leaving Fox with a 12-footer to stay alive.
'I've already holed this one, so how hard can it be?' said Knox, jokingly, to his caddy James Williams. "The putts have been going in the last couple of weeks, I had a great feeling on the greens and I just kept going". This is a heck of a start, but I feel like I've got a lot more to go.
On a tense and magical afternoon at Ballyliffin, the 33-year-old Scot finally got his hands on the trophy following a finish of his own that was so spectacular, it deserves to sit right alongside McIlroy's prodigious efforts when it comes to any highlight reel. I stood over the putt.
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He described how his mother was a trailblazer who went to law school, became a prosecutor and then a trial judge. The Democrats are now pressuring Republicans who support access to abortion services to oppose the nomination.
"I thought my chances had slipped on 17 to be honest, failing to make birdie", Knox said. "I hit the shots I wanted to down the stretch". He will now head to Carnoustie full of confidence and has thrown his hat in the ring for a place in Europe's Ryder Cup team. He'd already marked our cards with back-to-back T4s in the Scottish and Irish Opens previous year but this acts as a further reminder that he's a go-to guy whenever the European Tour heads for the links.
Defending champion Jon Rahm shot 66 to finish tied fourth with Van Rooyen, while Danny Willett continued his recovery from the depths in tied sixth spot.
Then in the play-off he had an nearly similar putt to extend the play-off but his effort lipped out and Russell Knox was able to celebrate a first Irish Open win. When Knox couldn't birdie the favourable par-five 17th, Fox struck, hitting the green in two and safely two-putting to take a one-stroke lead into the final hole.
Asked if he felt capable now of going on to win The Open, he said: "I do actually".
Wade Ormsby and Adam Bland were Australia's best, both finishing 15 shots behind the victor for a tie of 49th.