This match wasn't meant to be about the spinners. A pitch that started heavily grassed had the quicks on both sides queueing up for their turn ... or that was the theory at least. Instead, the two tweakers on show have proved key weapons over the first three days, with their business end of the contest still to come. Both Graeme Swann and Paul Harris have bagged hard-earned five-wicket hauls, and one of them could yet prove the match-winner in a finely balance contest.

Although Harris and Swann are opposing one other, they also share an understanding of each other's jobs. Spin bowling can be a lonely profession given that there is often just one such man in each team. They are expected to do the donkey work, tie up an end and not let the game slip away, but when conditions suit - as is the case here - they are expected to transform into match-winners. When Harris had Swann caught at deep square-leg to collect his fifth wicket and end a remarkable innings of 85, the two patted each other on the back in a sign of mutual respect.

"I always look out for the fingerspinners in the game," Swann said. "It is the hardest thing to do in Test cricket. People may not believe that, but it's a mugs' game. He was struggling by the end with only four wickets, so I was more than happy to do the best mate's thing and chip a catch to deep square-leg."

Harris, too, said the pair appreciate each other's skills. "We've got a nice little thing going. He's done very well and deserved his call-up first of all. He's played a lot of first-class cricket. We've played against each other quite a bit in the County Championship and we are good friends as well."

When Graeme Smith was asked whether he would consider omitting Harris in favour of four quicks, due to Jacques Kallis's injury, he said the spinner was 99% certain to play. The home side certainly weren't getting carried away by the surface and under hot, sunny skies the pitch has changed character dramatically. But the fact that spin has played such a role already has gone against the form book. "At one stage this deck was maybe a little too flat, but they have worked that out nicely," Harris said.

South Africa have come around to the value of a balanced attack in recent years and Harris played an important part in South Africa's rise to No. 1 in the world rankings. Yet he was nearly lost to the country after moving to England to take up a county career with Warwickshire, but was persuaded back when an international opening appeared. What a shrewd move that has proved to be.

He is rated as the ninth-best bowler in the world, three places above his opposite number, Swann, although both are now likely to receive further shunts up the ratings. Yet Harris is often overlooked when assessing South Africa's attack. It's more interesting to talk about Dale Steyn's pace or Morne Morkel's bounce. Left-arm spin isn't really that cool. But Harris deserves some headlines of his own.

Harris claimed perhaps the key wicket during South Africa's 2008 series win, when he tempted Kevin Pietersen to go for his hundred with a six at Edgbaston, and had him caught at mid-on. Had Pietersen batted for another hour or so South Africa could have been out of the game. Cleverly, Smith brought him on early on the third morning to bowl at both Pietersen's ego and his good friend and former team-mate, Jonathan Trott.

They were both fascinating duels. This time Harris didn't get Pietersen - although one delivery bounced inches over middle stump - but he did win the battle with Trott which gave him immense satisfaction. It was a wicket brought about by pressure as Trott became tied down and then tried an ugly heave over the leg side.

"We've been mates for a long time, that's no secret, and it's nice to bowl at him," Harris said. "He's done well - though I hope he doesn't in this series - and it's a bit weird to bowl at him. It was good to go one-up.

"With me and him, we are not mates on the field but are off it. I'm not going to not be mates with him because he's playing cricket against me. There are bigger things than cricket in this world. I'd like him to be my mate after cricket. I did have a few words with him, he took it well and just laughed it off."

Harris's next scalp came through batsman misjudgment as Ian Bell had a moment he'll want to forget. Harris isn't a huge spinner of the ball, but Bell clearly expected one to turn sharply when he shouldered arms and watched his middle stump go back. The ECB are investing in Merlyn spin-bowling machines for each country and Bell might want to spend time with Warwickshire's.

"I was surprised, there are two types of leave - a good one and a bad one," Harris said. "I think the ball before actually did it by spinning and hitting the splice and that put the doubt in his mind. I've been working on the straighter one, although some say that's all I have."

However, Harris then proved he could turn the ball by removing Paul Collingwood. It was the perfect dismissal for a left-arm spinner, drawing the batsman into a defensive shot on the front foot and finding an edge to slip. "It was nice to get one to turn," he said with a smile.

He tired towards the end of his stint - it was comfortably the warmest day of the series - but even after 37 overs and with the promise of many more in the second innings, Harris's job still wasn't finished for the day. With three overs to go, he had to emerge as nightwatchman following the dismissal of Ashwell Prince. Such is the lot of the spinner.