Poulter, who tipped Horsfield for success from the age of 13, has recorded victories on five continents during his illustrious career, but the Ryder Cup star has never won in his native England.
That prize was within Horsfield’s grasp as he took a one-shot lead into the final round at the Forest of Arden and the 23-year-old held his nerve to card a closing 68 to finish 18 under par, a shot ahead of Belgium’s Thomas Detry.
Detry had recorded his ninth birdie of the day on the 17th to move into the lead for the first time, only to bogey the last after seeing his par putt from three feet catch the edge of the cup and stay out.
That left Horsfield needing to play the last two holes in one under par and he produced a stunning approach to the par-five 17th, a towering fairway wood from 233 yards over the water setting up a two-putt birdie from just over the green.
A cautious tee shot on the last left Horsfield around 50 feet from the hole, but he safely two-putted for par to complete a closing 68 and one-shot victory.
“It’s special,” said the Florida-based Horsfield, who won the European Tour qualifying school by eight shots in 2017.
“With everything that’s going on in the world right now I’m thankful that the European Tour has been able to put on tournaments for us to play. I’ve been in Orlando for the last three months and I felt like my game was right there.
“I played decently last week (in the British Masters), had a pretty bad finish, but I kept the positives from that and played pretty solid all week.
“I was one back when I looked at the leaderboard on 16 and then hit one of the best five woods I’ve ever hit in my life on 17.
“Me and my caddie had a long discussion on the 17th tee because I wanted to leave myself a good five-wood number because it’s probably my favourite club in the bag. I had 233 yards, into and out of the left, and executed it perfectly.”
Horsfield is co-managed by Poulter’s caddie Terry Mundy and spoke to the latter on Saturday night for advice on how to approach the final round.
“We talked about him and Ian being in situations where they’ve had one-shot leads or having been leading at a golf tourna-ment,” Horsfield added.
“He said ‘go out there and play good golf, if you get beaten, you get beaten’. I had that mindset today. I thought I’m going to do my own thing, play good golf and if someone wants to catch me, then good for them.”
Detry, who won the World Cup for Belgium in 2018 alongside Thomas Pieters, said: “A lot of things have been going through my mind, but honestly I have nothing to be disappointed about right now.
“I played some really good golf today, the greens are getting trickier down the stretch and I over-read that putt on 18.
“But I’m very pleased with the way I finished on those last few holes to be honest. I hit some very good shots out there. Another missed opportunity, but I’ll move on and forget about it and just try to win next week.”
England’s Chris Paisley, Welshman Oliver Farr and Sweden’s Alexander Bjork shared third place on 14 under par. PA