London: Sri Lanka face a nervous wait to discover if captain Angelo Mathews will be fit for their Champions Trophy one-day international opener against South Africa on Saturday.

The all-rounder has been struggling with a calf problem and was due to have a fitness test on Friday.

But Sri Lanka are set to delay any final decision on Mathews until the morning of Saturday's match at the Oval.

"We're going to look at him tomorrow morning as well, so the final decision will be then," Sri Lanka vice-captain Upul Tharanga told reporters at the Oval on Friday.

By contrast South Africa, the world's top-ranked ODI side,  begin their quest to win the 'mini World Cup' bolstered by the knowing both leg-spinner Imran Tahir and batsman David Miller are fully fit for the Group B fixture.   

Left-arm spinner Keshav Maharaj proved an effective deputy when called upon by the Proteas during their recent 2-1 ODI series defeat by England but skipper AB de Villiers was glad to have Tahir available following a hamstring tweak.

"He's our number one spin bowler," said de Villiers.

"Keshav understands his role when it comes to that.

"There might be a time in the tournament where we might use a different strategy, if we play against a certain opposition or on a certain field.

"But for tomorrow, Imran is the number one pick.

"He is fit and ready to go, so you should see his name on the team sheet."

South Africa whitewashed Sri Lanka 5-0 in a home ODI series earlier this year but de Villiers said: "They always play well in these tournaments.

"We're well aware of that, and they deserve our respect.

"They have a lot of match winners on their team.

"So it's so much more important for us to make sure we don't think about what happened in the last series and we focus on tomorrow's game."

South Africa rounded off their recent series against England with a morale-boosting seven-wicket win at Lord's on Monday, having ensured all the members of their Champions Trophy squad featured at some stage.

"As difficult as it is for me to say ... that was definitely a warm-up for us," said de Villiers.

"I hate to say that -- because I hate losing games of cricket.

"It was a bit of a catch 22 to get that right message across to the boys, (and) it was maybe a bit confusing at times.

"But if you do go into a series thinking that you want to play all 15 players, you're not really focusing on just winning that series -- because your mind is a little bit elsewhere.

"I think that's what happened."

He added: "We actually could have won that series, easily -- which is a great boost of confidence for us -- keeping in mind that we played 15 players, and it was a bit of a warm-up for the Champions Trophy."