PREVIEW - South Africa vulnerable against improving Kiwis

DURBAN (Reuters) - Two successive series defeats, the loss of their captain and a dramatic drop from the top of the test rankings leaves South Africa vulnerable as they prepare to host New Zealand in a two-match encounter.

South Africa, the top-ranked test nation before defeats by India and England saw them slide to sixth, return to the five-day format at Kingsmead on Friday after a seven-month hiatus without injured skipper AB de Villiers and pace bowler Morne Morkel.

In their absence the hosts face a tough examination against a youthful and confident New Zealand side.

Kane Williamson, who has blossomed into one of the world's top batsmen, started his reign as New Zealand test captain with two easy wins in Zimbabwe in recent weeks. The matches on flat surfaces in Bulawayo gave the Kiwi batsmen plenty of time at the crease to prepare for a formidable South African seam attack that is likely to welcome back Dale Steyn after he injured his shoulder in December.

Also returning is Vernon Philander who was injured in November playing a game of football.

Kagiso Rabada, who took 13 wickets in South Africa's last test outing against England, is set to complete a three-man pace attack. "From a seam bowling point of view they are strong, they have got a lot of variety," New Zealand coach Mike Hesson told a news conference.

The touring team must decide whether to use two spinners to back up a pace attack of Tim Southee, Trent Boult and South African-born Neil Wagner.

"We wouldn't be afraid of playing two spinners if we felt the pitch was going to be dry and turn," added Hesson. Conditions are uncertain for what will be a first-ever test in Durban at this time of year, making it an even more difficult assignment for replacement captain Faf du Plessis.

He is back for South Africa after being dropped for the previous test against England in January. This will be the 15th series between the two countries. New Zealand have never won, with 11 South African victories and three draws.

(Writing by Mark Gleeson in Cape Town; Editing by Tony Jimenez)