Four of rugby's top openside flankers will feature in the two World Cup quarter-finals in Wellington this weekend. Wales captain Sam Warburton goes up against Ireland dynamo Sean O'Brien, current European player of the year, while Australian David Pocock faces a battle royale against Springbok fetcher Heinrich Brussow. The breakdown has increasingly become the key aspect of play in rugby union, and a proven exponent of the "tackle and jackal" is worth his weight in gold. An openside in defence must display a rare turn of speed, an ability to tackle and the strength and foresight to quickly rebound in a bid to snag the ball from the attacking side, or at least slow it down. Turnovers are their manna, instantly putting the other side on the back foot. In attack, the openside is expected to be happy as a ball-carrying weapon to punch holes in defences and also offer invaluable support to other runners, be it at the breakdown or at offload time. In Warburton, O'Brien, Pocock and Brussouw, fans will have the best of both the northern and southern hemispheres to cheer on, All Blacks skipper Richie McCaw -- in action in Auckland against Argentina -- notwithstanding. "It's a small battle within a large war," Wales assistant coach Shaun Edwards said of the likely tussle for dominance between Warburton and O'Brien. "They're different sorts of players. One of Sam's strengths is definitely on the floor, and he proved that when he played against Brussouw, he pretty much dominated the breakdowns in that game, getting six turnovers. "O'Brien's more a power runner, very explosive. In a year-and-a-half's time they'll be fighting for the number 7 shirt on the (British and Irish) Lions tour, maybe playing in the same backrow, who knows?" Experienced Australian backrower Rocky Elsom warned the breakdown would be key, especially in the knock-out phase of the World Cup. "If you ever discount the breakdown you're going to be in trouble, particularly as we get to the pointy end of the tournament," he said. "That's going to be an area of contention, because any team that gets the dominance at the breakdown they're going to provide front-foot ball for their attack and that is an enormous part of the game. "If you look at the teams whose attack has really stuttered, you can almost always link it back to the breakdown." Springbok flanker Brussouw said he rated Pocock, whom Australia sorely missed in their shock 15-6 pool loss to Ireland, as one of the best flankers around. "He has played solidly for his team for the last two years and has been important for his team," Brussouw said. "Pocock is not just a fetcher," he said. "He has other very good qualities. "Having a guy like him back is a huge confidence booster because he helps his team get into better defensive positions by slowing down the ball and stealing ball. "He's like me, in that we don't carry a lot of ball because we'd rather make sure we get to the rucks and Australia's main gameplan is based on quick ball." Springbok coach Peter de Villiers added: "We have two of the best players who play at the ball in world rugby out on the field with David and Heinrich so it's going to be a great contest. "They are very good at breakdown time, and that's where (Australia) get all their play from turnovers but we have to look after our ball carefully and put good phases together."