Henrik Stenson produced a storming finish to claim the lead at the halfway stage of the Nedbank Golf Challenge as Danny Willett's title defence was hampered by a back injury.

Shortly after play resumed following a two-hour delay due to a thunderstorm, Stenson trailed playing partner and local favourite Jaco van Zyl by three shots with seven holes to play.

However, the world number seven, who rated his chances of simply playing in the event at five per cent after spending three days in bed with the flu, rolled in a hat-trick of birdies from the 13th and picked up another shot on the 17th to add a 67 to his opening 66.

At 11 under par Stenson was one shot ahead of Van Zyl, who had started his round birdie, eagle before eventually settling for a 68, with American Robert Streb another shot back following a 66, the lowest round of the day.

“I’m surprised given that on Wednesday afternoon I didn’t think I would be able to tee it up,” Stenson admitted.

“I thought I was done and dusted, so to be leading after two days it’s a turnaround to say the least. But the saying is ‘Beware of the injured or sick golfer’ and I guess that’s true.

“I just tried to stay patient and get round and I’ve done that pretty well so far. I’m getting over the virus, but I was more fatigued this morning than I was yesterday just because of playing yesterday. It takes a lot out of you playing in 35 to 40 degrees when you’re not physically well.

“I’m heading in the right direction, but I’m not getting a chance to rest up much. Tomorrow I’ll probably have very heavy legs again, but being in the hunt for a tournament should get me going and hopefully that will kickstart it tomorrow.”

Stenson had followed three birdies in the first five holes with a hat-trick of bogeys from the sixth and admitted that the delay caused by the threat of lightning came at the right time.

The 39-year-old, who was on the 10th hole at the time, said: “I was not on a good run before the break. I didn’t do too much wrong but managed to string three bogeys in a row, so at that time it was probably good to have a break and it really cooled down afterwards for the back nine.

“I got the putter going – that was hot at least – and made a couple of nice birdie putts. All in all a very good back nine turned an average round into a good one.”

Willett began the day just a shot off the lead but ended it nine adrift after a 75 which featured a front nine of 40 at the Gary Player Country Club.

The 28-year-old was in obvious distress from the recurrence of a long-standing back injury and after playing the first five holes in four over, lay down on the sixth fairway and performed stretching exercises before receiving treatment from the European Tour physio on the ninth tee.

That certainly appeared to help as the world number 20 pitched to 15 feet on the par five for his first birdie of the day, and the weather delay which followed allowed him to receive further treatment before covering the back nine in 35.

Willett went for more treatment immediately after his round, leaving playing partner Van Zyl to reflect on his bid to win a first European Tour title and possibly climb into the world’s top 50 to secure a Masters debut in April.

“This tournament is one my dream events to play and if I do really well here it’s a means and a way to get into my other dream event, which is the Masters,” said Van Zyl. “So I’m very well aware of what’s going on around me here.

“It felt a little like a duel between me and Henrik today. I looked at the leaderboard and I think we were two or three clear of the rest of the field. It’s always good fun, especially if you’re in the same group. You can kind of keep an eye on each other.

“There’s good reason why he’s number seven in the world, looking at the putts he made coming in. I missed quite a few fairways today which made it a little bit tricky, especially after the rain delay, but overall it was not too bad.”

Ireland's Shane Lowry finished the day in a share of 22nd position on one over in the 30-strong no-cut event, carding a one-under 71 that included five birdies and four bogeys.