Woods has gone 14 years since he last won the green jacket, 11 years since his last major and had five surgeries - four on his back, the other to rebuild his left knee.
Brooks Koepka of the United States reacts to a missed birdie putt on the 18th green during the first round of the Masters at Augusta National Golf Club on April 11, 2019, in Augusta, Georgia. After a flawless opening round, Koepka had two bogeys and a double in his first six holes on Friday but the twice US Open champion was able to regain his composure, steadying himself with six straight pars from the ninth.
Day is tied for the lead at seven under alongside countryman Adam Scott (68), as well as fellow major winners Brooks Koepka (71), Louis Oosthuizen (66) and Francesco Molinari (67).
British Open victor Oosthuizen also joined the party with a 68, while world No 2 Dustin Johnson returned a two-under 70 who to join Woods at six-under.
Scott, the 2013 Masters victor, hit a 2-iron to within four feet for eagle at the par-5 15th hole on the way to a 68 to reach 7 under. He answered with three birdies on the next four holes, including a shot into the 16th that stopped inches from the cup.
"I missed a few putts out there, but I'm not too bummed out about it because I hit them on my lines, " he said.
"It is really an incredible leaderboard", Scott said.
Day said he had his chiropractor working tirelessly on his back on Thursday night and Friday morning to get him ready to play through the injury.
"I can't complain because she's birthed three children and I haven't, so she's a lot stronger as a person than me; I just hit a little white golf ball around a course".
He holed out from behind the green at No. 12, picked up birdies on the par 5s, and rolled in a bending 20-footer in between on the 14th.
The only scare for Woods came on the 14th hole, when his drive went well left into the trees.
Scott then knocked in the eagle putt to move to eight under par.
Koepka, who had shared a one-shot overnight lead, also deserved applause, staging a back nine recovery after a stumbling start to the second round.
"I've shot this number and won four coats, so hopefully, I can do it again, " said Woods, who slightly miscalculated.