India's all-conquering Test side will face its toughest challenge when it tours South Africa in 2018, according to former Proteas skipper Graeme Smith.

Virat Kohli's side has risen to the top of the world rankings after eight successive series victories, a run that stretches back two years

The likes of New Zealand, Bangladesh, England and Australia have all been brushed aside on home soil, while India has also enjoyed success on its travels.

Tours to Sri Lanka in 2015 and 2017 have produced comfortable wins - the latest a 3-0 whitewash - while it was a 2-0 winner in the West Indies in last year.

However, it is scheduled to travel to second ranked South Africa next year having never won a series there.

On five of its six visits India has been beaten - winning just two of 17 Tests - and Smith thinks its next trip will be a decisive step for Kohli's side.

"India have been doing quite well in Test cricket," Smith told the Hindustan Times.

"A lot can be attributed to how Virat Kohli has been playing Test cricket and inspiring team-mates. 

"I think the conditions in South Africa will be very different and western for the Indian team. 

"They have been playing either at home or in Sri Lanka and the Caribbean where the ball comes a bit slow. The conditions in South Africa will be the real test for the Indians."

He added: "It would be interesting to see how Indians fare against South Africa's bowlers like [Kagiso] Rabada, [Morne] Morkel, [Vernon] Philander and Keshav Maharaj.

"Also, having Dale Steyn back in the side would work for the South Africa team. He has done well against the Indians and having his experience would mean a lot for South Africa. 

"I think South Africa's batsmen will have to score big. The batting department looks a bit unsettled. Also, the combination South Africa chooses will make a difference."