Harrington shot 4-under 68 at St. Andrews, one of three different courses used over the first three days of competition, to finish 54 holes at 16-under-par 200. The Irishman is tied with Argentine Eduardo Romero, who shot 67.
'I missed only one drive out there today and that was the key,' said Romero. 'This is one of the best courses in Europe - its very similar to my home course in Argentina, no trees and plenty of wind.
Harrington, who hasnt won since the 2001 Volvo Masters, started the day with a one-shot lead, and was leading by that same count when he made 6 on the par-4 penultimate hole.
The Dubliner pulled his sand wedge approach shot from 105 yards just short of the greenside bunker. He then duffed his chip and three-putted.
But at the 18th, Harrington drove his tee ball into the Valley of Sin, just short of the green, and rolled up his second shot to four feet.
It was important to birdie 18 and that meant quite a lot to me, Harrington said.
A 68 is a good score and Im quite happy with it. But the reality is that 6-under ' or even better ' would have put distance between myself and the rest of the field.
Singh, like Harrington, drove his ball some 40 yards from the hole at 18. The Fijian, however, hit a bump-and-run that clipped the flagstick and fell into the cup.
Seventy-two players made the cut, which fell at 2-under 214. Those 72, along with 20 amateur teams, will play the final 18 holes Sunday at St. Andrews.
Full-field scores from the Dunill Links Championship