South Africa coach Rassie Erasmus has admitted he was impressed by likely quarter-final opponents Ireland’s opening Rugby World Cup performance.

According to Erasmus, the only side that produced a display comparable with Ireland’s in the opening round of fixtures was New Zealand, who beat South Africa 23-13 in their opening game in Pool B on Saturday.

The defeat left the Springboks on course for a last-eight meeting with Ireland in Tokyo on October 20, and Erasmus was paying close attention to Joe Schmidt’s side in their resounding 27-3 Pool A victory over Scotland on Sunday.

“Apart from New Zealand they were the only other team that put in a full 80 minutes of constructive, well-planned, decisive and clinical rugby (in the opening matches),” Erasmus said.

Rassie Erasmus was impressed with Ireland's display against Scotland Rassie Erasmus was impressed with Ireland's display against Scotland Rassie Erasmus was impressed with Ireland’s display against Scotland

“Physically and tactically they were really good, the same as New Zealand. Those are the two teams that I think have been really consistent in the last two years and they will both be really tough opponents.”

The South African coaching staff will be well aware of Ireland’s strengths and weaknesses if the two sides do meet in the quarter-finals.

Prior to taking over the Springboks, Erasmus and defence coach Jacques Nienaber spent almost two years coaching Irish province Munster, who have 12 players in Ireland’s 31-man squad.

Former Munster attack coach Felix Jones also joined the Springbok technical team four weeks ago after attack coach Swys de Bruin suddenly resigned for health reasons.

Felix Jones knows Ireland's tendancies very well Felix Jones knows Ireland's tendancies very well Felix Jones knows Ireland’s tendancies very well

Erasmus says Jones has already done valuable work for the Springboks in breaking down opposition defences.

“I sat down the players and we agreed we cannot change our attack philosophy now,” Erasmus said.

“They said they would rather like to get someone in to help us analyse the opposition’s defensive structures.

“It is something that we do not really need for the Southern Hemisphere teams that we play on a regular basis, but knowing we will probably play Scotland or Ireland, and maybe England and so on, I know how good Felix is at that.

“The analysis he does on individual players and team defensive structures is phenomenal. He has slotted in really well with the boys, he is a young guy and so associates with them really well.

Felix Jones won 13 caps for Ireland Felix Jones won 13 caps for Ireland Felix Jones won 13 caps for Ireland

“We will tap into his brain. He does a lot of work individually with the players to help them get better.

“In the Northern Hemisphere, there is a lot more focus on small details that we in South Africa take as brute talent and do not try to refine. That is something I want to change in our rugby.”

Jones played 13 times for Ireland, including a 29-15 victory in Dublin in 2014 in which six of the current Springbok squad played.

The 32-year-old also served as an assistant coach for Ireland during their tour Japan in 2017 and Schmidt admitted last month he is concerned by his presence in the Springbok camp.

Felix Jones worked under Joe Schmidt in Japan in 2017 Felix Jones worked under Joe Schmidt in Japan in 2017 Felix Jones worked under Joe Schmidt in Japan in 2017

“It’s awkward because you don’t have to be a rocket scientist [to know] he came to Japan with us last time so he was right in amongst it,” Schmidt said.

“He’s seen everything that we deliver and he would have a great knowledge of even the language we use in our camp so it’s awkward for us.”