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Even in friendlies such as this, Duff refuses to give anything other than his best as he teased, tormented, shimmied and jinked his way past opposition who are renowned for a swashbuckling style of play.
But for all his attacking flair, Duff further underlined the fact he has grown in stature during his time at Stamford Bridge, as Kerr has witnessed since a millstone £17million move from Blackburn in the summer of 2003.
The 25-year-old is no longer the kind of player content to sit in an opponent's half and wait for the ball to come to him.
Instead, he tracks back and defends, as was often the case in this match, never more so when he produced a tackle on his own dead ball line to keep Portugal at bay.
That was primarily the theme of the game as Ireland as a team denied Portugal the time, space and opportunity to string passes together, belying the fact they came into the game having scored 20 goals in five matches in their World Cup qualifying campaign to date.
Coach Luis Felipe Scolari's side, beaten by a hard-working Greece in the European Championship finals last summer, again found themselves undone by a degree of industry.
They were restricted to set-piece plays and long-range shooting for their chances, and instead it was the Republic who were the more creative, even if their only goal was initiated from a corner.
But they made their intent clear in the fourth minute when new Spurs winger Andy Reid was allowed to charge unopposed into the area, and although an angled shooting chance was on, he opted for a pull back that was cleared by Petit.
Then Duff showed how fired up he was when he was embroiled in a private battle with Cristiano Ronaldo that perhaps spoke volumes for the current rivalry at the top of the Barclays Premiership between Chelsea and Manchester United.
Duff went in hard, but fairly on Ronaldo, but it prompted a verbal exchange, a bout of shirt pulling and then a strong word from Premier League referee Matt Messias before they reluctantly shook hands.
Portugal, sporting black and white shirts in support of the Stand Up Speak Up campaign against racism, then conjured two chances in the space of four minutes.
Chelsea midfielder Tiago struck a superb 22-yard drive that flashed inches wide before Jorge Andrade somehow ballooned a header over from underneath the bar as he rose unopposed at the far post to meet Ronaldo's inswinging corner.
But Duff responded, attacking the right wing, taking players on and delivering his own far-post ball to Robbie Keane for a header that angled back into the area rather than the back of the net, with the Spurs striker claiming he had been pushed.
After Reid again unselfishly opted for a pass rather than a shot inside the area after being set up by Kevin Kilbane, it at least led to the 21st-minute corner that sparked the goal.
Reid was the supplier with an inswinger for Manchester United defender John O'Shea to flick on for O'Brien to head home his first international goal on his 18th appearance past Ricardo from six yards.
Ireland were certainly bossing the game, although on the half hour Shay Given pulled out his first save by beating away a swerving free-kick from Petit who followed that with another bullet drive that was painfully blocked by O'Brien.
Duff almost closed the half with a goal he would have deserved, almost wrong-footing Ricardo with a swerving shot of his own that the Portugal goalkeeper only just managed to cling onto.
The game inevitably lost its sting after the interval as the substitutions unfolded, although there were still moments to savour.
There was Duff shaking off the attentions of two Portuguese defenders with a piece of pure skill before delivering a dinked cross to the far post where Robbie Keane was denied by one of Portugal's new faces in Ricardo Costa.
Late on Costa then struck his own right-hand post as Keane chased a loose ball, while there was a standing ovation for Duff when he was replaced in the 69th minute.
He had previously blotted his copybook by earning the game's first yellow card for a clip on the heels of Ronaldo, before being on the receiving end soon after when tackled by Rogerio Matias, who was also cautioned.
Finally, there came a heart-stopping moment in the 86th minute when former Newcastle star Hugo Viana struck a stunning low drive from 25 yards that had Given beaten.
For a split second it appeared as if it had clipped the inside of the post, with the ball rippling the back of the net, but instead it had struck the stanchion and rolled behind.
But in terms of starting a vital year, with World Cup qualification at stake, Ireland could not have done any more to keep Portugal quiet, with Duff the silencer-in-chief.