From where did you get these percentages of winnings for both teams ?
Looking at the odds, things are quite different as shown in the above percentages. I don't think that South Africa chances of winning this match are 61%. The odds suggest that they have more than 90% chance of winning.
By the way, if anyone wants to take risk and bet on USA, they will get above 5 odds. The form they're in at the moment, you never know another upset may happen and also South Africa was lucky in some of their wins in the group stages.
The South African team will surely perform well in this match, as they are still marked as the strongest team of this season. South Africa team performance is very good because they have batting line and bowling line can attack widely. USA team players are very experienced but relatively less strong than South Africa team, because in past matches South Africa team's margin of victory was more.
There is definitely a huge difference between these two teams as per the betting in this match, although the win in this match should not be decided but based on the team margin I would say South Africa team has a 79% chance to win.