Thomas, the champion at Nine Bridges two years ago, had nine birdies, including four in the first four holes, and no dropped shots to move to 13 under after two rounds.

That left the American two shots ahead of first-round leader An Byeong-hun, who followed up a 64 with a 69.

“I played well,” former world number one Thomas said. “I didn’t do anything unbelievably. It just was a very stress-free day.

“Obviously playing well, that makes it stress free, but I just kind of kept the ball in play off the tee, hit some good irons.

“I think I hit every green and made a couple putts early. And then the back nine, I got a few close to the holes where those birdies came easier. So it just was a very easy 63 if you can somehow say that, but hopefully we’ll be able to keep some of that going for the weekend.”

Korean-born New Zealander Danny Lee is tied for second with An after a 66. It would have been one better but for a bogey at the par-five 18th.

American Jordan Spieth and Argentina’s Emiliano Grillo are two shots further back in a tie for fourth. Spieth was one of the big movers of the day thanks to a 65.

French Open

At the French Open in Paris, Gavin Moynihan is in a tie for 10th place on three-under par going into his second round.

George Coetzee and Kurt Kitayama share top spot on six-under.

Under pressure to retain his European Tour card for next season, Paul Dunne is six-over par.

Continental Europe’s oldest national open has moved from its traditional summer spot in the schedule to the autumn and the softer conditions and lighter rough made scoring conditions favourable at the venue that was so formidable at last year’s Ryder Cup.

There were 49 players under par at the end of round one and Coetzee and Fox led the way after carding rounds of 65 each containing a single bogey.

Scotland’s Richie Ramsay was a shot off the lead at five under alongside local favourite Benjamin Hebert and American Kurt Kitayama.

South African Coetzee set the target in the morning and he reached seven under at one point before a bogey on the 17th dropped him back.

“It was almost perfect,” he told europeantour.com. “I played really well. It’s nice to be in the mix again. It’s been kind of a hard, long year but it feels like the game is coming along nicely.”

Fox’s single blemish came on the 13th but he redeemed himself with a stunning tee-shot on the par three 16th to share the lead.

“I’m pretty happy,” he said. “This is a golf course that can kick you pretty quickly. I played really solid today, holed some putts.

“I played those tough holes, that closing stretch, really well. I could have potentially grabbed a couple more on those last four, which is always a bonus, but very happy to walk away with a 65 around here.”

Ramsay has already safely secured his playing privileges for next season after taking it right to the wire in 2018, and he enjoyed his opening 18 holes in the French capital.

“It’s obviously a fantastic golf course,” he said. “A little bit different this year. A bit colder, a bit softer, so the course is playing quite long.

“It’s a joy to play a golf course like this because it just tests the whole part of your game and mentally, obviously there’s a lot of water out there, so you have to be very committed to your shots.”

One man who did not enjoy the day was 2012 champion Marcel Siem, who disqualified himself after a rules mix-up.

The German incorrectly believed preferred lies were in play and lifted, cleaned and replaced his ball five times on the fairway in his first round before discovering his error and walking off at the turn.

“I played the ball from the wrong place five times and conceded 10 penalty strokes,” he wrote on Facebook. “That was a little too much for me and I disqualified myself.”