Lahiru Kumara struck twice as Sri Lanka's bowlers had the better of a rain-hit opening day of the first Test against South Africa at Kingsmead on Wednesday.

South Africa were 80 for four when rain stopped play for the day shortly before lunch. Fast bowler Kumara took wickets in successive overs to put his team on top – and was denied a third wicket because of a no-ball.

"There was a lot in favour of the bowling team and they bowled well. The deliveries that got wickets were all pretty decent," said South Africa batting coach Ashwell Prince.

"It's a skilled bowling line-up and obviously they have done their homework," he added.

Sri Lankan bowling coach Darshana Gamage said pace was Kumara's biggest asset. With a history of hamstring injuries, he is now being used as a Test specialist.

"In this Test championship, we wanted to have all our bowlers fresh," said Gamage.

"Our biggest challenge is to have quicks who will win us Test matches and I can say that we have the nucleus now to do the job away from home," he added.

Sri Lanka's decision to bowl first on a well-grassed pitch paid off under a heavily overcast sky.

Namesakes Ajitha and Vishwa Fernando dismissed both South Africa's openers with the total on 14.

Tristan Stubbs and South African captain Temba Bavuma defended doggedly, taking the total to 29 after 13 overs.

There was a flurry of scoring when Kumara came into the attack in the 14th over, with 17 runs added off his first over – four leg-byes, a no-ball and three boundaries by Stubbs.

But the strongly-built Kumara found extra bounce with the first ball of his second over, having Stubbs caught at third slip for 16.

The weather is expected to clear from Thursday and Prince said South Africa were in a reasonable position, referring to the first Test between Australia and India in Perth last weekend when two low first innings totals were followed by a strong Indian second-innings performance.

The two-Test series is crucial to both sides' hopes of reaching next year's World Test Championship final at Lord's, with Sri Lanka sitting third in the table and South Africa fifth. (AFP)