Mancini Wins; W. Germany Out of Davis Action : Tennis: Anti-apartheid activists threaten to demonstrate at semifinals in Sydney if three Australian players compete in South Africa.

Defending champion West Germany was ousted from this year’s Davis Cup competition today when Alberto Mancini of Argentina beat Carl-Uwe Steeb 7-6 (8-6), 6-3, 6-4 in the decisive match in Buenos Aires.

Mancini needed only 25 minutes to close out the match suspended Sunday because of darkness.

Leading two sets to none with the third set tied at three games apiece, Mancini lost his serve, broke Steeb’s serve, held his own and won all four points off Steeb’s serve in the 10th game.

Argentina now plays Australia in the Davis Cup semifinals in September in Australia.

In Melbourne, Australia, meanwhile, anti-apartheid activists threatened today to demonstrate at the Davis Cup semifinals if three Australian tennis players compete in an exhibition event in South Africa this week.

Wally Masur, Darren Cahill and John Fitzgerald, all of whom played in Australia’s 3-2 World Group quarterfinals victory over New Zealand in Brisbane, left for Johannesburg to take part in a doubles tournament.

David Howes, spokesman for the Australian Anti-Apartheid Movement, said they should be barred from representing Australia and warned that his group would stage protests against them.

Australia will play either Argentina or West Germany in the Davis Cup semifinals at White City in Sydney Sept. 21-23.

Howes expressed the group’s outrage at the players’ decision to compete in South Africa, which is ostracized from most international sports because of its government’s discriminatory policies.

“Their mercenary action demonstrates the most appalling insensitivity to the current situation in South Africa,” Howes said. “We hope that even at this late stage it will be possible for the non-racial sports movement in South Africa, the National Sports Congress, to convince the trio not to compete in the tournament.

“Should this prove unsuccessful, we will certainly consider all options available to us to register our protest against the players.”

Australian Cup captain Neale Fraser said the choice of whether to play in South Africa is a matter for individuals.

“They are well aware of the problems associated with it, but they are off,” Fraser said.

By playing in South Africa Masur, Cahill and Fitzgerald will forfeit their right to play for Australia in the Olympic Games.

Another Australian Davis Cup team member, Mark Kratzmann, played a challenger event in South Africa last year, and former Wimbledon champion Pat Cash won the South African Open in Johannesburg in 1987.