Cricket is obviously a team sport with defined roles for all the team members, but like other sports, certain players are more important to their teams than others.

Barend Prins takes a look at the key players in the upcoming One-Day International series between England and South Africa.

South Africa:

Graeme Smith:

Smith's leadership and ability with the bat are crucial to South Africa's cause, but I do not have anything to add to Tim Human's ? piece that was run earlier this week on iafrica.com.

Jacques Kallis:

Nearing the end of a glittering career and not without his critics, Jacques Kallis, in my opinion, is still South Africa's best and most important player. He has at times been called a reluctant bowler, but it is his role as the fourth ? and sometimes third ? seamer that adds vital balance to the team. When Kallis does not play, his prolific run-scoring and experience is obviously missed, but in Hashim Amla the Proteas seem to have found an able deputy at the top of the order. More often than not an extra fast bowler would have to be played though, weakening the batting line-up lower down the order in the process and thus weakening the team. Basically, Kallis fulfills the role of two players (not to mention his bucket-like hands at slip) and is quite simply irreplaceable.

JP Duminy:

Fast becoming one of the most exciting cricketers in the world, Duminy's remarkable ability to 'change gears' depending on the situation is what puts him in the upper echelon of players currently plying their trade. The fact that he is also a very handy off-spinner ? and known to be something of a partnership-breaker ? is what makes him an integral part of the South African line-up already, despite cementing his place in the team during the last season only.

Arguably the best fast bowler in the longer form of the game at present, Dale Steyn has not quite been able to replicate his Test form in the ODI arena. With Makhaya Ntini seemingly no longer considered an option in the ODI side, and with the experienced Charl Langeveldt not guaranteed a spot either, Steyn is now the undoubted leader of the Proteas' relatively young bowling line-up. He is the one the likes of Wayne Parnell, Ryan McLaren and Albie Morkel will look to for some inspiration when they need it. If Steyn is unable to cope with this extra responsibility, we might be in for a long summer.

England:

Like Graeme Smith, Strauss's leadership and ability at the top of the order will be integral to England's hopes throughout the summer ? not just in the ODI Series. More than that, we cannot say.

Like him or not, Kevin Pietersen is England's best player by an absolute mile and the one most likely to win a match on his own. Brash and arrogant but also quite brilliant, his record in South Africa is one to behold ? scoring 454 runs in six innings, including three centuries at the quite incredible average of 151.33. He is bound to get his fair share of boos around the country, but if his previous tour to his homeland is something to go by, it will only spur him on more. Not that it did his team a whole lot of good (in the ODI Series) last time around...

Paul Collingwood:

England's most experienced one-day player and the one you would expect to perform in a pressure situation. If the South African bowlers take early wickets regularly ? which is fairly likely ? Collingwood's gritty determination makes him the player England will look to to get them out of a tough situation. If he fails however, England will struggle to put South Africa under any sort of pressure during the series.

Jimmy Anderson:

Very hard to play when there is some swing about, Anderson is England's most experienced bowler on tour and the one most likely to trouble the strong South African batting line-up. That also means that if he has a wayward series, the likes of Kallis, Smith and De Villiers will have a field day against England's otherwise tame bowling attack.