New Zealand were far from perfect in their Rugby World Cup opener, but a six-minute spell of breathtaking brilliance showed why the two-time defending champions remain the team to beat.

While the All Blacks are understandably many people's favourites to prevail in Japan, it is fair to say their status as rugby's dominant nation has come under more questioning than usual in recent months.

South Africa won this year's Rugby Championship, in which Steve Hansen's men suffered a stunning 47-26 defeat to Australia - albeit one they avenged the following week with a 36-0 thumping of the Wallabies to retain the Bledisloe Cup.

The All Blacks have also been displaced by Ireland at the top of World Rugby's rankings and the likes of Wales and England have been tipped to mount strong challenges for glory over the coming weeks.

For 20 minutes on Saturday, any doubts over New Zealand's strength were heightened as South Africa started superbly in a blockbuster Pool B opener in Yokohama that could well serve as a dress rehearsal for the final at the same venue on November 2.

Throughout the first quarter, the Springboks dominated territorially, their ferocious line-speed causing no end of problems.

However, one misplaced pass from Faf de Klerk - with South Africa leading 3-0 - triggered a sudden shift in momentum as the All Blacks demonstrated a ruthless streak no other team can match.

The Boks initially got off lightly when De Klerk's wild pass from a ruck was pounced upon by Richie Mo'unga, who kicked ahead before being halted just short of the line. Makazole Mapimpi was penalised for not releasing the number 10, but the wing avoided a yellow card and the All Blacks merely picked up three points to level the score when a try had appeared highly likely.

New Zealand had found their mojo, though, and in a matter of minutes they took complete control with two stunning tries in quick succession.

Hansen's decision to start Beauden Barrett - widely viewed as the world's finest fly-half - at full-back in order to accommodate Mo'unga has prompted much debate, but the presence of two playmakers was certainly influential as the All Blacks hit top gear.

Mo'unga's cross-field kick to Sevu Reece started the move that led to the first try. Reece promptly skinned the out-of-position Mapimpi before working the ball through Aaron Smith to a galloping Ardie Savea. From the next phase, Barrett surged through a gap from second receiver and laid on a simple finish for George Bridge.

Barrett was also heavily involved in the passage of play that led up to the All Blacks' second score, which was finished by namesake Scott Barrett after Anton Lienert-Brown had dazzlingly weaved past five defenders.

Having been 3-0 down and on the rack, New Zealand were suddenly 14 points to the good and their opponents understandably appeared somewhat stunned for the remainder of the half.

To their credit, the Springboks did battle back gamely after the interval, but they had been left with too much to do.

It has been 12 years since the All Blacks last lost a World Cup fixture. Their magical six-minute blitz on Saturday suggests it will take something special to deny them another title.