Ten years ago 25-year-old McIlroy's dad Gerry and three friends each placed a £100 bet at odds of 500/1 that his son would become Open champion before he turned 26.
That would mean a windfall of £200,000 in total for the quartet if McIlroy can turn a six-shot lead into the third major title of his career.
The Northern Irishman has won each of his two majors by eight shots, the first in the US Open at Congressional in 2011 and the second in the US PGA Championship at Kiawah Island the following year.
However, in 2011 McIlroy also took a four-shot lead into the final round of the Masters at Augusta before collapsing to a closing 80, while he himself won the BMW PGA Championship at Wentworth this year from seven shots behind with 18 holes remaining.
How Rory ruled the roost on Day Three
The predicted thunderstorms failed to materialise, but Rory McIlroy provided the fireworks to take a stranglehold on the 143rd Open Championship at Royal Liverpool. A "significant risk" of storms and an amber weather warning led the R&A to employ a two-tee start for the first time in championship history on Saturday, with play getting under way at 9am from the first and 10th tee.
But after a spell of heavy rain ended in time for the leaders to tee off, McIlroy repelled a spirited challenge from fellow 25-year-old Rickie Fowler with two stunning eagles in the last three holes.
A round of 68 gave McIlroy a 16-under-par total of 200 - just two shots outside the all-time Open record set by Tom Lehman in 1996 - and a six-shot lead over Fowler, who also returned a 68. Spain's Sergio Garcia and American Dustin Johnson were a shot further back on nine under.
McIlroy will now attempt to become the second wire-to-wire winner of a major in succession after Martin Kaymer won the US Open at Pinehurst last month by eight shots, the same margin by which McIlroy won the 2011 US Open and 2012 US PGA Championship.
And if the Northern Irishman succeeds in lifting the Claret Jug, the winner of the Open will have completed three legs of the career grand slam for the second year running.
Phil Mickelson's victory at Muirfield means he needs to win the US Open to join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods as the only men to have won all four majors.
For McIlroy, the Masters would be the only trophy missing from his collection, the tournament he led by four going into the final round in 2011 only to collapse to a closing 80.
McIlroy also led the 2010 Open at St Andrews after an opening 63 only to shoot 80 in the second round in atrocious conditions. Four years on he benefited from being on the right side of the draw with an early start on Thursday and late start on Friday, a day which had caused him so many problems this season.
In 2014 he had been 50 under par in the first round and nine over in the second until Friday's six-under 66 saw him match Woods' halfway total of 132 from 2006.
The 72 players who made the halfway cut - including Woods right on the mark of two over par - had been sent out in groups of three rather than two, with defending champion Mickelson alongside past and present US PGA champions Jason Dufner and Keegan Bradley in the first match out.
McIlroy, looking to become just the third man in the modern era after Woods and Nicklaus to win three majors by the age of 25, began his third round just after 11am and immediately saw his four-shot advantage cut in half.
The former world number one, who had carded consecutive rounds of 66, found a greenside bunker with his approach to the first and a poor escape led to just his second bogey of the week, while playing partner Dustin Johnson holed from three feet for birdie.
With heavy rain having stopped by the time the leaders teed off, Johnson was not the only player to take advantage of the softer greens, Fowler picking up four birdies in his first six holes before dropping a shot on the seventh.
McIlroy registered his first birdie of the day by two-putting the par-five fifth to get back to 12 under and crucially saved par from 20 feet on the seventh after finding heavy rough off the tee and only being able to hack out onto the fairway.
At that stage he led by three from Fowler - the only player to finish in the top five in both majors this season - but a thrilling duel between the 2007 Walker Cup rivals was about to develop.
McIlroy saved par on the ninth as well after pulling his tee shot left of the green, where it hit a buggy-mounted camera and bounced off the leg of the cameraman.
After making sure the man was unhurt, the buggy was moved and McIlroy pitched to eight feet and rolled in the putt, while up ahead Fowler birdied the 10th and 11th before McIlroy responded from seven feet on the 11th.
Fowler then picked up his seventh birdie of the day, and third in a row, from five feet on the 12th and that was soon good enough for a share of the lead when McIlroy bogeyed the same hole after twice tangling with the heavy rough.
It was Fowler's turn to find trouble off the tee and bogey the 14th and McIlroy was suddenly two ahead once more when he holed from 35 feet for an unlikely birdie on the 14th.
Two ahead became five ahead in the space of 15 minutes on the par-five 16th, Fowler taking six after a poor drive and McIlroy holing from 18 feet for an eagle after a superb drive and towering long-iron approach.
Fowler, Garcia and McIlroy all bogeyed the 17th from similar spots left of the green, but while Garcia parred the last and Fowler made birdie, McIlroy stamped his authority on the event with another eagle from 10 feet.
Tiger Woods rues missed opportunities
Three-time Open champion Tiger Woods admits he is still making too many mistakes as he continues his comeback from back surgery.
The American's 73 on the third day of the 143rd Open Championship, for a three-over total, was only his fifth competitive round since an operation in March.
He opened with successive birdies, having begun at the par-five 10th due to the R&A's decision to hold a two-tee start because of the threat of bad weather, but could not maintain that momentum and a second nine of 38 included a triple-bogey seven after losing his ball in a gorse bush, as well as a double-bogey six.
"I've made a lot of mistakes. I've made two doubles and two triples (this week) but on top of that I missed a lot of shots for opportunities for birdies and consequently I'm three over par," Woods said.
"I'm starting to get the flow of the round, the flow of playing again, but still I've just made too many mistakes.
"You can't run up high scores like that and expect to contend, especially when the conditions are this benign.
"Most of the scores are three under par or better. I certainly didn't do that."
Woods wore a black ribbon, as did many of the players, in memory of coach Bob Torrance, father of Ryder Cup-winning captain Sam, who died of cancer on Friday.
"I've known him a lot over the years, even from my amateur days when I came over here. He's been fantastic to me," Woods added.
"We've had a lot of great conversations about golf. It was always fun picking his brain about the game, how it's evolved and how he believes it should be played.
If you have a complaint about the editorial content of the Belfast Telegraph or Sunday Life then contact the Editor here. If you are not satisfied with the response provided then you can contact the Independent Press Standards Organisation here
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? firstname.lastname@example.org