South Africa captain Faf du Plessis believes his side will be able to play freely as the underdogs in the Cricket World Cup opener against England.

The tournament gets up and running on Thursday at The Oval, with hosts and top-ranked England taking on Du Plessis' Proteas, who are without star bowler Dale Steyn due to a shoulder injury.

South Africa sit third in the world ODI rankings and arrive on the back of a six-match winning run in the 50-over format but Du Plessis insists his side are clear underdogs and that it is England who most cope with the pressure of being favourites.

"Whether you are favourites or not, you still have to play good cricket," Du Plessis told a news conference.

"[England] deservedly have the tag because they are the home nation and have consistently played good cricket.

"England are the favourites so if it means on the day there is less pressure on us then we can play freely.

"We are going in as underdogs and if that releases some players in the team then that is great."

Du Plessis' counterpart Eoin Morgan, meanwhile, is relishing the favourites tag ahead of what will be his 200th ODI appearance for England.

"We have spoken about it as a group, the level of expectation and the favourites tag is there for a reason," Morgan said.

"Over the last two years, particularly at home, we have been outstanding and that is the reason it is there. 

"There is a lot of belief within the dressing room. We are confident with our own game. Dealing with it is a challenge in itself but one we are looking forward to. A win under any circumstances will do."