Nations sides in World Cup action 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. “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.