Joe Root would rank a series win over South Africa as his greatest achievement as England Test captain and believes the "sky is the limit" for his young side.

England head into the fourth and final Test at the Wanderers - which starts on Friday - with a 2-1 lead, having already extended their unbeaten run in away Test series against the Proteas to four.

The tourists have been hit hard by both illness and injuries in South Africa, with James Anderson, Jack Leach and Rory Burns having all returned to England, while paceman Jofra Archer missed the last two Tests with an elbow issue.

Given the circumstances, Root says beating the Proteas in their own backyard would probably go down as his most satisfying accomplishment since taking over as skipper almost three years ago.

"It's been a tour that has thrown everything at us," Root told reporters. "But the whole squad, the players and the coaching staff, have worked tirelessly to make sure we stayed calm and as in control as we could.

"When we've had our opportunities to bounce back, we've really taken them. It's been really pleasing in many ways.

"We're a very young side at the start of something and we've seen some very promising performances. To win three Tests in a row would be a very big achievement for this group.

"A series win here probably would be my proudest achievement as captain, because of the different things we've had to manage throughout this tour.

"We've had injuries and senior players having to fly home and a lot of illness to deal with and we have adapted to very different conditions.

"For a young group of players that has been very pleasing and a sign we're doing the right things. If we can harness that and keep looking to improve then I do think the sky's the limit for this team.

"But we're very much at the start of something here and we're not perfect by any means. We have to keep that attitude and keep driving that forward for a long period of time if we want to get to number one in the world."

Mark Wood came into the side for the third Test and it remains to be seen if he will be able to play back-to-back matches with Archer pushing for a recall.

"It would be nice to see how Archer and Wood go together," Root said.

"It would be a relentless barrage of pace. It would be great to have that extra bit of pace and firepower, but ultimately we've got to hit the right areas as we have done in the last couple of games."