Defending champions South Africa beat a hard-hitting Tonga 49-18 in Marseille on Sunday to bag a crucial bonus-point victory that leaves them atop Pool B and with one foot in the World Cup quarter-finals.

The Springboks now have 15 points from their four matches, but the pool winners and runners-up who advance to the quarter-finals will only be decided when Ireland -- who beat the Boks 13-8 last week -- take on Scotland in Paris on Saturday, with a number of permutations possible.

After that defeat by the Irish, the Boks knew they had to win against Tonga, with a bonus point, and preferably by a large score to improve their points differential.

A victory it proved to be, the Boks crossing for seven tries but the Tongan side stayed competitive throughout and went over for three of their own.

"We knew they weren't just going to lay down and give it to us. We made a couple of mistakes but we fought to the end - really proud of the guys' effort," said Bok captain Siya Kolisi.

Kolisi exchanged jerseys with Tongan counterpart and try scorer Ben Tameifuna, who added: "South Africa are the best in the world and it's always tough to cross their line and we managed to do that a few times tonight."

Handre Pollard was back in the driving seat for the Springboks for the first time in 13 months, the fly-half who booted 22 points in the 2019 World Cup final win over England kicking four conversions from four attempts before going off on the 50-minute mark with the game already won.

Tongan counterpart William Havili opened the scoring with a third-minute penalty for offside, but Pollard spurned a shot himself as the Boks went for touch in the search for tries.

That duly came when Cobus Reinach tapped and went, the scrum-half diving in at the corner.

The Boks were then put under a 10-minute spell of real pressure from the Pacific Islanders, with cries of "Tonga! Tonga!" bellowed out by the partisan crowd at a packed Stade Velodrome.

Having withstood the barrage, the green and gold machine moved confidently back up the field, and once in Tongan territory, Canan Moodie latched on to a loose ball that ricocheted off Vincent Koch's head to scoot in under the posts.

- Clinical finishing -

Tonga repelled a rolling maul close to their line, but a succession of infringements saw them unable to break free and hooker Deon Fourie ended up being driven over.

Tonga got their just desserts, however, when skipper Tameifuna crashed over, the man mountain of a prop using every ounce of his 150kg (23st 6lb) frame to dot down. Havili missed the extras to leave it at 21-8 at half-time.

After such a doughty defensive performance and throwing everything into attack, the question was if Tonga, who lost 59-16 to Ireland and 45-17 to Scotland in their two opening games, might run out of gas in the second period.

Jesse Kriel, on for Makazole Mapimpi who was forced off after a clash of heads with Augustine Pulu, ensured the Boks' bonus point in the 50th minute, burrowing over after a series of forward charges.

Pollard converted for an unblemished kicking record, immediately replaced by Manie Libbok.

But Tonga were not done, winger Fine Inisi crossing for his team's second try after a great miss-out pass from Havili.

Ex-Wallaby Adam Coleman made an immediate impression when he came on, disrupting an attacking Bok line-out.

But when Pita Akhi coughed up the ball on a midfield crash from the resulting scrum, the ball was moved quickly to the left wing where Willie le Roux was on hand to crash in at the corner.

Libbok put any doubts over his goal kicking to bed by nailing the tricky conversion and then adding extras to a try by Marco van Staden.

Tonga replacement Patrick Pellegrini made the most of the luck of the bounce as he gathered a chip to cross for a deserved third try, while Kwagga Smith had the last word, notching up the Boks' seventh try.

lp/dj