Paul Casey leads as Dustin Johnson closes in at Valspar Championship

Paul Casey remains on course to become the first man to retain the Valspar Championship title after carding a three-under 68 in the third round at Palm Harbor in Florida.

That lifted the 41-year-old Englishman into a one-shot lead on nine-under 204 going into Sunday's final round on the Copperhead Course at Innisbrook Resort.

Casey, who is bidding for a third PGA Tour victory, has never previously defended a title during his career and, after edging out Patrick Reed and Tiger Woods last year, he will need to repel the challenge of world No 1 Dustin Johnson this time around.

Johnson heads the chasing pack on eight under after birdieing the 18th for a 67, with his fellow American Jason Kokrak one shot further back and Luke Donald tied for fourth on six under.

Casey started the day tied for the lead with Austin Cook, although they found themselves one behind a charging Louis Oosthuizen as they played the opening hole.

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A birdie at the first put Casey alongside the South African, although he immediately bogeyed the second before getting back on track with a superb approach shot to six feet to set up a birdie at the sixth.

Oosthuizen suffered in 'The Snake Pit' as he bogeyed the 16th and 18th over the tough final three holes and Casey was able to take charge on the back nine with birdies at the 11th, 12th and 14th thanks to some superb iron play.

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Casey held a three-shot lead entering the closing stages of his round but had his advantage reduced to one when he followed Johnson's birdie at the 18th with a bogey at the same hole when he drove into a fairway bunker and missed his par putt from 16 feet.

Johnson, who had missed the cut on his two previous appearances in the tournament and had not been back for nine years, took a share of the lead with three birdies in four holes from the fourth before bogeying the ninth.

He was flawless on the back nine, holing a 33-foot putt for birdie at the 14th despite twice visiting the rough, and then playing a superb, towering shot from a fairway bunker to set up his birdie at the last as he recorded his 14th consecutive round in the 60s.

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Kokrak, who is seeking his first PGA Tour win, stormed into contention by coming home in 30 for a five-under 66, following three birdies in five holes with a perfectly-judged hole-in-one at the 15th.

Donald, the 2012 champion and playing in just his second tournament of the year, played some consistent golf to remain in the hunt, topping and tailing his one-under 70 with birdies, while his only bogey came at the sixth.

The Englishman shares fourth with Scott Stallings (70), with Oosthuizen (66), Cook (72), Jim Furyk (68), Canada's Nick Taylor (67), Curtis Luck of Australia (70) and South Korean Sungjae Im (71) tied for sixth place on five under.

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Scotland's Russell Knox moved up to three under thanks to a four-under 67, but Jon Rahm could only manage a 72 as he bogeyed the 16th and double-bogeyed the 18th to slip down to two under.