Justin Thomas vaulted into a two-shot lead at the halfway stage of the CJ Cup in Korea after firing a superb nine-under 63 in the second round.
Thomas birdied the first four holes and managed to build on his hot start, holing his ninth birdie putt of the day to climb to 13 under par overall, two ahead of first-round leader Byeong Hun An and Korean-born New Zealander Danny Lee.
Jordan Spieth enhanced his prospects of a first win since The Open in 2017, but defending champion Brooks Koepka went in the other direction as a succession of errors contributed to a 75 which left him 13 strokes off the pace and outside the top 50.
There were no such problems for world No 5 Thomas, the winner on Jeju Island in 2017, as he followed his four-birdie flyer with another gain at the eighth and three in five holes after the turn.
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Thomas kept the mistakes at bay and atoned for his bogey-six at the 18th on Thursday with a solid birdie to close out a 63 which he described as “effortless”.
“That was, I would say, overall one of the better rounds I’ve played in a while,” he said. “It came very easy. I felt like all the putts I made really weren’t anything crazy.
“The putt on 10 was probably the hardest one I had, but for the most part, I had them in the right spots for where they were fairly easy putts or at least closer to the hole. I never really had to worry too much out there, which is definitely a bonus.”
An slept on the overnight lead and began day two with a nervy bogey at the 10th, but the home galleries were soon cheering as he recovered with a birdie at the 12th and then made four in a row from the 17th, lifting him to 12 under with seven holes – and two par-fives – to play.
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But the former BMW PGA champion struggled over the final third of his round and dropped his second shot of the tournament at the sixth before parring in for a 69 to stay at 11 under, a score later matched by Lee after he took a little gloss off his 66 with a bogey at the last.
Four birdies in six holes from the 12th lifted Lee to 12 under, but he caught a wedge heavy from inside 100 yards at the last and came up short of the green, a mistake which led to a closing six.
“I had a 95-yard third shot, it was downhill, and I was trying to hit my 60-degree wedge a little too hard and hit behind it a little bit,” said Lee. “Got short-sided and I didn’t make the up and down. But other than that, it was a very positive round.”
Spieth, still chasing his first victory since lifting the Claret Jug at Royal Birkdale, recovered from a bogey at the first with four birdies on the back nine and, after another blemish at 11, he birdied five of the last seven holes to return a 65 which propelled him to nine under along with Emiliano Grillo (66).