The run propelled Harrington to sixth place in the PGA Tour FedEx Cup standings, giving him a chance at an $11.35m (€7.74m) payday this weekend at East Lake Golf Club.

Harrington must win the $1.35m (€920,000) Tour Championship and see FedEx Cup leader Tiger Woods finish no better than third in the 30-man event if he is to carry off a $10m (€6.8m) bonus as FedEx Cup champion and he certainly began the right way in Georgia, reaching four under par at the halfway stage of his first round on the par-70, 7,154-yard East Lake course.

That gave the Dubliner an early share of the lead with American Sean O’Hair, who had played 10 holes of his opening round.

Open champion Stewart Cink, who began the tournament with only a slim chance of landing the big bonus prize from 26th place in the standings, had been first to hit the front at four under after nine holes in his home state.

One birdie followed on the back nine, book-ended with bogeys at the 13th and 15th holes and Cink reached the clubhouse with a 67 at three under, with Scott Verplank on the same mark after 11 holes.

US Open champion Lucas Glover and South Africa’s Retief Goosen were two under after 16 and 14 holes respectively with Steve Marino in the clubhouse following a 69 alongside six fellow Americans including Phil Mickelson, Jim Furyk and Steve Stricker, still out on the course at one under.

A win for Woods, Stricker, second in the standings, and Furyk, third, would guarantee them the FedEx Cup, as it would fourth-placed Zach Johnson and fifth place Heath Slocum.

Yet FedEx Cup standings leader Woods, looking to add a sixth title of the year, was off to a poor start.

The world number one had began steadily and birdied the third hole but then bogeyed both the sixth and eighth to fall to one over par after eight.

England’s Luke Donald, the only other European in the field besides Harrington, carded a level-par 70.