09/17/2011 07:25 EDT | Updated 11/17/2011 05:12 EST

Irish fire up World Cup by beating Australia; Springboks crush Fiji. Pumas rip apart Romania

HAMILTON, New Zealand - Of the Tri-Nations sides in World Cup action on Saturday, one was said to have the best backline in the world and the other was old and slow.

Who would have thought then that by the end of the day, it would be defending champion South Africa dazzling with a power-packed 49-3 win over Fiji, while Australia's vaunted backs would be smothered in a 15-6 defeat to an impassioned Ireland in the tournament's first major upset.

In the day's first match, Argentina rebounded from an opening loss to England by beating Romania 43-8.

The Irish, who couldn't win any of their four warmup matches last month, outsmarted and outsmucled the Wallabies in the rain at Eden Park to fire up the tournament and shake up the playoff possibilities.

If Ireland holds on to top Pool C, it will likely avoid meeting the Tri-Nations sides early in the playoffs. If No. 3 South Africa finishes atop Pool D and No. 2-ranked Australia finishes second to Ireland, the southern hemisphere teams will meet in a quarterfinal, with the winner likely to advance to a semifinal with top-ranked New Zealand.

Not only did Ireland rekindle the flame which ended England's Six Nations Grand Slam bid in March, but it also turned into a title contender with its second win from two Pool C matches.

Behind a dominant scrum and fearsome loose forward trio, Jonathan Sexton and Ronan O'Gara each landed two penalty goals and Sexton added a drop goal to inflict the Wallabies' first pool-stage loss since 1995.

James O'Connor kicked two first-half penalties for Australia, which conceded five scrum penalties and struggled to display any fluidity after having lost flanker David Pocock and hooker Stephen Moore to injury or illness in the hours leading up the match at Eden Park.

"It was the performance we knew we had in us," Ireland captain Brian O'Driscoll said. "We had incredible support and it was our job to give them something to shout about, and thankfully we managed to do that."

The Springboks impressed for the first time all year. They'd been criticized for being one dimensional and aging, and were admittedly fortunate to edge Wales last weekend, so it was a surprise when they uncorked a brutally efficient performance to nullify the physical Fijians and score six tries without conceding any.

"It was good to get our plan in place and to be able to execute well against a team that was willing to play rugby," Springboks captain John Smit said.

South Africa endured a fiery opening from Fiji, who ran the ball with typical vigour from all over the field only to smash into an imposing defence. Fiji's own line made the South Africans work for every meter, but the Boks used quicker ruck ball to stretched Fiji to breaking point, frequently.

The win — South Africa's third from as many meetings with Fiji — followed its 17-16 win over Wales six days ago and kept it unbeaten atop Pool D with matches against Samoa and Namibia remaining. Fiji dropped to 1-1.

"The Springboks probably showed why they're world champions," Fiji captain Deacon Manu said.

Smit's 108th test gained a share, with lock Victor Matfield, of the South Africa record for most test appearances. Matfield missed Saturday's match with a hamstring strain, as did centre Jean de Villiers with a rib cartilage sprain and winger Bryan Habana with a knee injury.

The win was South Africa's ninth in succession in the World Cup, approaching the record of 10 it set in 1995 and 1999.

Argentina scored four tries in the first 30 minutes against Romania in Invercargill in the early game, showing plenty of the attacking flair lacking in its opening loss to England.

The Pumas pack was again dominant, but this time the backline was potent to lead 26-8 by halftime.

Santiago Fernandez, Juan Manuel Leguizamon, Juan Figallo and Lucas Gonzalez Amorosino, voted man of the match, scored the first-half tries, converting Argentina's glut of possession. In the second half, replacements Juan Jose Imhoff and Genaro Fessia scored tries. Center Martin Rodriguez kicked five conversions and a penalty for 13 points.

Winger Iionel Cazan scored the lone try for Romania, while Tiberius Dimofte kicked a penalty.

Meanwhile, U.S. captain Todd Clever escaped suspension when he was cleared of a dangerous tackle charge in the win over Russia. The ruling means Clever is available to play against two-time champion Australia on Friday in Wellington.

Also, the All Blacks headed to Christchurch on a four-day goodwill trip to the city devastated by two major earthquakes in the past year. The entire squad will be in New Zealand's damaged second city, which was stripped of all of its World Cup games, until Wednesday when it will return to Auckland to prepare for France next weekend.

"Christchurch has showed us a lot of support and we want to show them that we care and the whole of New Zealand cares for that city," centre Conrad Smith said. "The lift we get out of it and the lift Christchurch gets out of it will be well worth it."

On Sunday, Wales faces Samoa in coach Warren Gatland's hometown of Hamilton, England plays Georgia at Dunedin, and France tackles Canada in Napier.