Jason Day is back on his feet and back in contention at a major.

Playing his first tournament since last month's U.S. Open, where he collapsed because of vertigo, the ninth-ranked Australian birdied three of his first six holes and picked up three more shots in the tougher back nine to join the crowd on 6-under 66 at the British Open on Thursday.

Day found a cool day on the links of St. Andrews much less of a grind than a round in the heat and hills of Chambers Bay.

"I'm not thinking about falling over on my face again," Day said. "I feel healthy, and I'm good to go."

The three-week break has done Day some good. He was exhausted by the end of the U.S. Open, during which he fainted due to vertigo in the second round, then dragged his body through the third round despite continued bouts of dizziness.

It wasn't the first time Day had to deal with the condition and he knows it won't be the last. He said he was taking medication that was helping, and feels fine in his bid for a first major title.

"If it comes, it comes," he said. "I can't really control it."

Day was in full control on Thursday until he scrambled for pars on Nos. 17 and 18, completing a bogey-free round that was the lowest in a high-profile group containing Tiger Woods and Louis Oosthuizen — the last two winners of the British Open at St. Andrews.

It's no surprise to see Day's name on the leaderboard. He has eight top-10 finishes in 19 appearances at majors, and has been runner-up on three occasions.

Behind the world's top two of Rory McIlroy and Jordan Spieth, and a rejuvenated Dustin Johnson, Day is delivering a consistent challenge.

"They just won't go away," said Day, looking at a leaderboard topped by Johnson on 7 under and with Spieth lurking on 5 under. "It's good to see because it's good for the game of golf.

"It's kind of extended on from the U.S. Open with Dustin Johnson and Jordan Spieth where they are right now, so it's going to be an exciting three days coming up."