Mendis (76), Asalanka (79) & Shanaka (68) hit defiant, rapid fifties
But Sri Lanka are eventually bowled out for 326 to lose by 102 runs
South Africa post 428-5 - highest total in World Cup history
Markram smacks century off 49 balls - fastest in Men's World Cups - before falling for 106
Van der Dussen and De Kock also hit sublime centuries
Rassie van der Dussen and Quinton de Kock content to tough it out for the time being.
Dilshan Madushanka with some disciplined lines. His final ball, round the wicket to the left-handed De Kock, hits the South Africa opener on the pad.
It was full from the Sri Lanka seamer but he contains his appeal. Nothing to suggest it was worth a review.
Steven Finn
Former England bowler on BBC Test Match Special
Look at the strength of South Africa, especially the middle-order Markram, Klaasen and Miller, they will have a part to play. I really like watching Kagiso Rabada bowl when he's in full flow. The toss may make it more of an even contest, but by money is on South Africa.
Kasun Rajitha backs up that over from Dilshan Madushanka with a maiden over as Sri Lanka attempt to put the brakes on South Africa.
Sri Lanka haul it back a little as Dilshan Madushanka tightens his lines and South Africa are only able to pick up a couple of singles off the over.
Here's the dismissal of South Africa skipper Temba Bavuma from a little while ago.
In Dharamsala, a pretty ropey Afghanistan batting performance saw them muster only 156 against some very canny Bangladesh bowling.
Bangladesh have lost their first wicket in the reply - Najibullah Zadran's direct hit from point running out Tanzid Hasan. The Tigers are 24-1 after six.
England play Bangladesh at the same ground on Tuesday.
South Africa respond to the early loss of their captain with a brash counter attack, and four consecutive fours.
Rassie van der Dussen is off the mark with a flick of his pads from Dilshan Madushanka then benefits from a thick edge for another down through fine leg.
Quinton de Kock also ups the ante at the start of Kasun Rajitha's second over with two thumps through extra cover.
Steven Finn
Former England bowler on BBC Test Match Special
It looked from the naked eye like it was going down leg, but it just snuck back in.
Bavuma lbw b Madushanka 8 (SA 10-1)
Temba Bavuma is on his way back to the pavilion.
It's full from Dilshan Madushanka and the Proteas skipper was pinned a fraction above the knee roll attempting to work the ball of his legs.
The ball-tracking technology shows it would have straightened to hit the top of middle.
Jonathan Agnew
BBC cricket correspondent on Test Match Special
Bavuma looks like he doesn't like that very much - he's going to review it. I thought I heard two noises, it might have been two impacts on the pad.
Dilshan Madushanka with the second over for Sri Lanka and a let off for Quinton de Kock who spoons a pull shot into the air, but it plops safely over the fielders in the ring. Temba Bavuma follows it up with another boundary as he cuts off the back foot.
Bavuma is then struck high on the pad by Madushanka's fourth ball and is given out on the field. The South Africa skipper reviews it.
This looks close.
Matthew Henry
BBC Sport in Delhi
The TMS team has successfully made the trip from Ahmedabad to Delhi.
The box in Delhi is, rather interestingly, behind a screen at the back of the press box.
Kasun Rajitha with the first over and Quinton de Kock is fairly watchful first up, eventually working the Sri Lankan seamer's fourth ball of his legs for a single.
Proteas skipper Temba Bavuma makes a lovely start as he crisply drives his first ball through the covers for four. That was pleasing on the eye.
Matthew Henry
BBC Sport in Delhi
The only press box in the world with a fountain?
There's a darker type of soil in Delhi and a fair bit of grass on the pitch at the Arun Jaitley Stadium so spin might not be as much of a factor as you would typically imagine.
We've just finished the national anthems and the players are readying themselves for the start of play.
South Africa: Quinton de Kock (wicketkeeper), Temba Bavuma (captain), Rassie van der Dussen, Aiden Markram, Heinrich Klaasen, David Miller, Marco Jansen, Gerald Coetzee, Keshav Maharaj, Kagiso Rabada, Lungi Ngidi.
Sri Lanka: Pathum Nissanka, Kusal Perera, Kusal Mendis (wicketkeeper), Sadeera Samarawickrama, Charith Asalanka, Dhananjaya de Silva, Dasun Shanaka (captain), Dunith Wellalage, Kasun Rajitha, Dilshan Madushanka, Matheesha Pathirana.
Matthew Henry
BBC Sport in Delhi
This stadium, with its stand that looks like it has been hit by an earthquake, has always fascinated me on the TV. I’m very excited to be here.
It’s packed full of character. I’ll have more on that shortly.
Sri Lanka captain Dasun Shanaka has won the toss and - as has been the pattern in the first two games - decided to field first.
Image source, Getty ImagesDay three of the Cricket World Cup and we take a closer look at couple of the dark horses as South Africa take on Sri Lanka in New Delhi.
South Africa come into the tournament off the back of a series victory over Australia and are second in the ODI rankings, although the loss of fast bowler Anrich Nortje to injury is undoubtedly a blow.
Sri Lanka, meanwhile, reached the Asia Cup final last month and have the benefit of experience of subcontinental conditions even without key all-rounder Wanindu Hasaranga, another injury absentee.
Welcome to our live text commentary from the Arun Jaitley Stadium.