Centre Bundee Aki says Ireland "weren't as clinical as we wanted to be" despite a bonus-point win over Tonga at the Rugby World Cup.

Andy Farrell's side, ranked number one in the world, face reigning champions South Africa on Saturday.

"I know we got the points that we wanted but we're still not happy with how we performed," Aki said.

Ireland, who beat Romania 82-8 in their opener, recorded a second consecutive bonus-point win in Nantes on Saturday to continue a perfect start to Pool B, where South Africa and Scotland are tipped to push the Six Nations Grand Slam winners for a place in the knockout stages.

Despite scoring 20 tries in their opening two matches, player of the match Aki said his side needed to improve before facing the Springboks in Paris on Saturday.

"Full credit to the boys and full respect to Tonga. They came out firing and we knew it was going to be a physical match," added Aki, who has scored four tries in Ireland's first two matches.

"Much respect to them, hopefully they go out over the next couple of weeks and leave everything out there.

"I think there are a lot of things we can work on. We weren't as clinical as we wanted to be. There were a lot of dumb penalties, you could say.

"Next week is going to be an even bigger test for us but we will go out there and give it our best."

'Sexton deserves record'

Head coach Farrell said he "thought it was a good performance" but joined Aki in saying his team needed to improve in aspects ahead of the South Africa match.

"A performance is never perfect," Farrell told ITV.

"Even when we were coughing up ball down on Tonga's try line, I thought we were well in control.

"I thought when we got a couple of scores up then we started hitting out straps. I thought it was a good mental performance. We stayed strong and true to the task."

Farrell also praised captain Johnny Sexton, who became Ireland's all-time points scorer with a try-scoring performance against Tonga, in which he scored 16 points before being substituted at half-time.

"He deserved to have it like that," Farrell added on the try that helped Sexton surpass his fly-half predecessor Ronan O'Gara's 1083 points for Ireland.

"The points, he would say that is his job to kick the goals for Ireland and score some tries now and again, but that doesn't make him.

"What makes him is how he leads his men every single week, and how he is selfless in how he gets his team ready to play.

"That is what he will be remembered for."