Lynn carded a 65 to climb into second place. He is two strokes ahead of Stephen Allan, who is minus-8, after firing a second straight 68 on Friday. Steve Webster is one shot further back at 7-under.
Casey, who started the day with a one stroke lead, birdied the first hole to get to minus-8. The 2001 Scottish PGA Champion gave that stroke back with his first bogey of the day at the fourth. He erased that mistake with a birdie at the next.
Casey posted his first eagle of the tournament at the ninth to climb to 10-under. He followed that up with a birdie at the par-5 10th. The 25-year-old faltered again at the 11th with his final bogey of the round.
The Arizona State graduate capped his round with a pair of birdies starting at the 14th. After that, Casey parred the final three holes to maintain his lead.
'Today was a lot tougher than yesterday. I was very happy with the way I played,' Casey said. 'There were a couple of fruity pin positions out there. They were just flat out good pin positions.'
Lynn posted his first birdie at the par-5 second. He later birdied Nos. 5 and 7 on the front side, before birdieing the ninth to make the turn at minus-7.
He continued his hot play on the back side with a birdie at the 10th. Lynn wrapped three birdies around his lone bogey of the round later on the back side.
Lynn birdied the 14th before faltering at 15. He came right back with birdies at 16 and 17 to climb into second place by himself. Despite the solid round, Lynn does not think he is at the top of his game.
'To be honest, my game is not feeling too great at the moment but I am hitting decent shots,' said Lynn. 'I do feel like my puttings good. I feel like I can hole anything.'
Niclas Fasth, Soren Kjeldsen, Richard Lee, John Bickerton, Thomas Bjorn and Ian Poulter are tied for fifth at 6-under 138. Poulter posted the best round of the day with a 64. He carded seven birdies and an eagle through his first 14 holes.
Peter Fowler, Ian Garbutt, Peter Lonard, Arjun Atwal and David Smail are one stroke further back at 5-under.
Defending champion Ernie Els, who has already won twice this season and was runner-up in Singapore last week, shot an even-par 72 and is tied for 37th at 2-under 142.