The 31-year-old has suffered a string of injuries since switching codes - making his Saracens debut in September as a blindside flanker before his first start at inside centre against Leicester earlier this month.
And with both of England's midfield slots well and truly up for grabs after the departure of head coach Andy Robinson, Farrell insists the time is right to throw his hat into the ring.
He said: "After two or three games at centre I don't think you can say that I am definitely going to the World Cup.
"Everyone in the Premiership wants to play in the World Cup. Everyone who is English wants to play in the World Cup.
"I'm happy playing centre. I'm playing there for Saracens and I'm enjoying myself - that's the main thing.
"I enjoy playing there and I'm looking forward to playing with some world-class players (on Sunday).
"I'm looking forward to playing at this type of standard - it's nice to be on the big stage again. It's exciting. We are all looking forward to it.
"You have got to play well in every game and the weekend will not be any different."
The struggling Springboks went down 32-15 to Ireland at Lansdowne Road before succumbing 23-21 to England at Twickenham and turning the tables in a 25-14 victory the following Saturday.
Bob Dwyer's World XV boast the likes of ex-England captain Lawrence Dallaglio and Wales number eight Michael Owen along with the third most-capped All Black of all time - scrum-half Justin Marshall.
And Farrell insists they are ready to fight fire with fire.
He said: "South Africa will be a physical test but we can match them.
"South Africa over the last four to five weeks have been developing and turning into a good side.
"We have to come together in five to six days and put in a good performance that will do us justice.
"We had a bit of a blip at the weekend (at Saracens) but hopefully we can move forward from there."
Saracens went down 20-17 at home to rock-bottom Worcester on Sunday - with fellow-centre Kevin Sorrell having already earmarked Farrell as the man to solve England's midfield malaise.
And fellow Saracens player Thomas Castaignede also claimed Farrell should banish once and for all any notion he has of playing in the back row.
"Andy was a good find from rugby league and he is definitely a centre - since the beginning I thought that and when he was playingÂ in the back row I was surprised," said the French utility back.
"Centre is where he should be and I think he will improve there. He could become the next Will Greenwood to bring the confidence to the team and he can organise the team.
"A lot of people in this team have things to prove and Andy has got everything he needs to be successful.
"He's got character, strength and the physical abilities. He just needs to practise at the highest level to play against the best teams and he has the chance to do that this weekend."