Jason Day is fit and firing once more, admitting he is "not thinking about falling over on my face again".

The 27-year-old, who collapsed during the second round of the US Open last month, doesn't know when his next bout of vertigo will creep up on him again.

But it doesn't seem to have deterred the Australian who, in his first tournament since his fall at Chambers Bay, rocketed his way back up to the top section of the leaderboard.

The Queenslander fired an impressive six-under 66 at St Andrews in the opening round, overshadowing two former Open Championship winners in Tiger Woods and Louis Oosthuizen to finish the opening day one shot behind leader Dustin Johnson and a stroke ahead of tournament favourite Jordan Spieth.

"I feel good, I feel good. I should just stay healthy so I don't have to answer these questions anymore," Day joked after his round.

"Look, I feel good. I'm not thinking about falling over on my face again. It's good. I'm not worried about it, I feel healthy and I'm good to go. I know what the process is when it comes to getting vertigo because the first time I had it was 2010. So I was just explaining earlier, if that was the first time I ever had it at the US Open, then I'd be kind of scared coming out here knowing maybe it's going to come back or I don't know yet.

"But speaking with my docs and being on this antiviral medicine that I'm on right now definitely helps. I know if I have it, it takes a couple of days to get rid of and obviously it's not great to have it in tournaments but if it comes, it comes. I can't really control it, that's the unfortunate thing."

Johnson, Spieth and Day were also among the leading contenders at the US Open, with Day admitting it was good for the sport.

"It's amazing isn't it, bloody hell," Day said.

"They just won't go away. It's good to see, it's good for the game of golf when Dustin Johnson is playing well, Jordan Spieth - [myself] every now and then. The only person we're missing is Rory [McIlroy]. It's unfortunate to see him out right now but hopefully he has a speedy recovery.

"Right now it's good excitement for the game of golf with how things are going. It has kind of extended on from the US Open with Dustin Johnson and Jordan Spieth kind of where they are right now. It's going to be an exciting three days coming up."

LEADERBOARD

-7 Dustin Johnson (U.S.)
-6 Paul Lawrie (Britain), Jason Day (Australia), Robert Streb (U.S.), Retief Goosen (South Africa), Zach Johnson (U.S.), Danny Willett (Britain)
-5 Ashley Chesters (Britain) 15 holes, Jordan Niebrugge (U.S.), Kevin Na (U.S.), Louis Oosthuizen (South Africa), Jordan Spieth (U.S.), Charl Schwartzel (South Africa)
-4 David Howell (Britain), Luke Donald (Britain), Matt Jones (Australia), Greg Owen (Britain), Branden Grace (South Africa) 15 holes, Marc Warren (Britain), Rikard Karlberg (Sweden) 12 holes

OTHER AUSTRALIANS
-2 Greg Chalmers, Marc Leishman, Adam Scott, Steven Bowditch
-1 Brett Rumford, Geoff Ogilvy, Scott Arnold
E John Senden
+1 Marcus Fraser
+2 Scott Hend
+3 Adam Bland 
+4 Scott Strange
+5 Rod Pampling