South Africa's David Miller praised Afghanistan as one of the surprises of the World Cup but admitted that his team's focus had already shifted to their semi-final clash with Australia.

South Africa qualified for the last four with six wins and two losses in eight games, while Afghanistan, their opponents in Ahmedabad on Friday, still have an outside chance of reaching the semi-finals.

"They've been really, really good to watch," said Miller of Afghanistan, who have beaten defending champions England as well as former winners Pakistan and Sri Lanka.

"They're a team that's certainly growing in confidence," he added. "They've shown that they can compete."

Miller also hailed Glenn Maxwell's unbeaten 201-run masterclass against South Africa in the group stage, which pulled Australia back from the brink of defeat.

"Seeing a knock like that was really special," said Miller of Maxwell, whom he will face again in the semi-finals next week.

"He's an X-factor, a match winner. It was inspiring to see no foot movement there and still hitting sixes and fours."

Miller admitted that players were excited at the prospect of a World Cup semi-final against Australia.

When they met in the group stage at Lucknow four weeks ago, Maxwell was out for three, and South Africa romped to a 134-run victory.

"We played them before in the World Cup, so it was kind of a dress rehearsal there, and we've played them quite a bit in the past," said Miller of the five-match home series on the eve of the World Cup that South Africa won 3-2.

"So, we know what to expect. They're always a competitive team. They've got some incredible players and match winners amongst their team."