The Springboks had been dire in their narrow opening win over Wales, but their direct running and suffocating defence proved too good for the South Sea Islanders, who only registered a penalty from Seremaia Bai.
Tries from Gurthro Steenkamp, Jaque Fourie, Francois Steyn, Morne Steyn, Tendai Mtawarira and man-of-the-match Danie Rossouw sealed the bonus point success for Peter de Villiers' side, which put them top of the group, with Morne Steyn booting 16 points for a personal haul of 21.
The groundwork was laid by an opening 40 minutes that yielded a 21-3 lead and Smit told the tournament website: "We had some work to do and we put in a quality first 40 (minutes).
"It was what we needed and I'm happy to keep a team like Fiji off our try-line."
But coach De Villiers warned his side that they still have to step up their efforts if they are to defend their crown.
He told reporters: "Fiji dominated the first 20 minutes and we missed five or six tackles that gave them momentum in the game.
"We can't allow that going forward. We pride ourselves on our defence and it worked pretty well in our favour but we must be ruthless on defence."
Fiji had run the Springboks close in their quarter-final meeting four years ago and had high hopes of doing so again, but they were comprehensively outmuscled and captain Deacon Manu acknowledged they had had no answer to the world champions.
He said: "It was a difficult game today and South Africa showed why they are world champions.
"They played with intensity for 80 minutes and it was a difficult old time for us."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.