Duff will pose a big threat to Puyol.

By Jamie Gardner, PA Sport

Mick McCarthy's Republic of Ireland team are one game away from matching their best World Cup finish when they reached the quarter-finals in 1990.

Standing in their way this time are Spain, so often World Cup underachievers but a team who look as though they are finally living up to their potential in this tournament, having come through their group with a 100 percent record.

Here we look at the key battles which could decide which of these two teams progress to the last eight.

DAMIEN DUFF v CARLES PUYOL

Duff's electric pace, most often utilised down the left-hand side, could spell trouble for Puyol.

The Barcelona defender is keen on getting forward himself, but may have to curb his forays down the right if Spain are to keep a close eye on Duff.

The Blackburn striker, along with Robbie Keane, appears to pose the biggest threat to Spain, and if Duff drifts out wide it will be Puyol's responsibility to track him.

ROBBIE KEANE v FERNANDO HIERRO

Keane has been a lively presence up front for Ireland in this World Cup and McCarthy will rely on his youth and enthusiasm to keep the Real Madrid defender busy.

Hierro, now 34, has plenty of experience but may struggle to live with Keane's pace, and if Ireland do opt for a more direct approach with Niall Quinn in tandem with the Leeds striker, Hierro will have to be on his guard.

And should the game go to penalties, you can bet on both of these two stepping forward to have a go.

MATT HOLLAND v JUAN CARLOS VALERON

The Ipswich midfielder has raised his profile considerably in the Far East, and he must be at his combative best to snuff out the creative threat of Valeron.

Much of Spain's attacking play goes through the Deportivo La Coruna playmaker and Holland must close him down quickly.

Ireland are never going to outpass Spain, so it is vital that Holland, their chief scrapper in the absence of Roy Keane, has a good game.

GARY BREEN v RAUL

This match-up seems to sum up the gap between the teams - the journeyman former Maidstone, Peterborough and Birmingham defender versus the Real Madrid superstar.

Their pedigrees may be very different, and Breen will probably have to play the game of his life to keep Raul quiet for 90 or 120 minutes.

Raul has it all - pace, technique and tremendous finishing ability and one slip from Breen, or conversely, one moment of Raul genius, could settle this game.