US Open Champion McIlroy carded seven birdies in his round of 68 at Woo Jeong Hills Country Club but also found the water twice in an eventful first 18 holes as the Northern Irishman finished in a tie for third on three-under.
"I felt like I played well. I started nicely and I was three under through nine holes, but I had a bad couple of holes on the back nine at 10 and 11 with a bogey and a double bogey," he said.
"I did well from there to birdie four of the next six holes to get back to four under, but then I was disappointed to bogey the last. But overall, it is a good start and something to build on.
"The shot on 11 was just a bad shot, while on 18 the wind grabbed it. I was aggressive, but I want to play aggressively."
Fowler, meanwhile, carded six birdies and two bogeys to set the early pace before the American was matched by defending champion Yang.
"I played well and drove the ball well which is important. I made a few birdies, but unfortunately I didn't play the par fives as well as I had hoped and played them two over, so that is something to obviously look to improve," said Fowler.
"I hit poor drives on those holes and on this course you have to drive the ball well to score. The score would have been really low if I had played them better, but it is something to improve on."