Justin Marshall: The Springboks' biggest threat

South Africa's captain Fourie Du Preez, right, celebrates after scoring a try during the Rugby World Cup quarterfinal match between South Africa and Wales. Photo / AP

As the All Blacks prepare for a massive semifinal against the Springboks, I believe their halfback and captain Fourie du Preez poses the biggest threat.

After his move to Japanese club Suntory in 2012, he's the one player that the All Blacks haven't seen a great deal of in recent times. While the All Blacks and Springboks usually play each other twice a year, he hasn't played Steve Hansen's men since 2013, a rip-roaring 38-27 defeat for the Boks in Johannesburg. Before that it was in Port Elizabeth in 2011.

During his time at the Boks he has been considered as one of the best halfbacks in the game. He's achieved a great deal, including winning a World Cup in 2007, so he's been to the coalface plenty of times. He knows what it takes.

He hasn't been playing for the Boks lately so for the All Blacks he changes the dynamic of his side. He's very good at controlling the game and he doesn't just do it with his kicking game. He has a running game which we saw last weekend against Wales when he scored the match-winning try after a nice assist from No8 Duane Vermuelen.

Du Preez understands the game well and he has such a wide variety of skills. He is good with the ball in hand and is prepared to try things. In the Wales game he picked the ball up from the base of the scrum and cross kicked to wing JP Petersen, who nearly scored. That takes vision and the good thing about him is that he is confident enough to try things.

The Boks haven't had that controlling influence for a while now. But, having said all that, I'm sure the All Blacks have done their due diligence on him.

Another aspect to this fascinating battle is that he has played Aaron Smith in a test only once - that victory over the Boks at Ellis Park in 2013, considered one of the best tests of all time.

Neither will know too much about each other and you've probably got the two best scrumhalves in the game matching up. If I was in du Preez's experienced boots, I would be looking to rattle Smith at every chance, because he is the key cog in the All Blacks' machine. If you can distract him and get under his skin, that could be detrimental to the All Blacks.

However, just as the All Blacks can't afford to focus on du Preez, he can't afford to spend too much time worrying about Smith.

The one area of relative weakness could be his passing game. It's not his greatest strength. If the All Blacks can put him off his stride so he doesn't get rhythm at the breakdown, that might an area where they can get an advantage.

The battle of the No9s will be intriguing at Twickenham; a victory for either one could go a long way towards a victory for their teams.