Watson (70 off 47 balls) carried on with the devastating form he showed against India and with help of the experienced Michael Hussey (45 not out), got the team home in 17.4 overs. Earlier, South Africa recovered to make 146 for five after Australia opted to chase in a crucial Super Eight contest at the Premadasa Stadium.
Australia are on the brink of making the last four after two victories in as many matches while South Africa suffered their second successive loss.
