Day was out of sorts on Thursday in posting a 1-over 71 but despite an obvious back injury and a delayed start because of weather he came out firing on day two with four birdies in his first five holes.
After going to the turn 5-under and just one off the lead, the third suspension of the day was called due to storms in the area and the former World Number One cooled on the back nine when play resumed.
Two dropped shots against a lone bogey added up to 66 for Day who will begin the weekend four off the pace set by American Jimmy Walker.
The reigning PGA Champion posted six birdies and dropped only one shot in his second round 65 to lead by two from Belgium’s Thomas Pieters and three over Zach Johnson, Rory McIlroy and Hideki Matsuyama.
Pieters will be fuming with his finish after squandering a birdie opportunity from six feet at the 18th to narrow the gap to one.
Day is the best of the six Australians competing in the no cut, US$9.75 million event, Adam Scott and Scott Hend two strokes further back and sharing 13th position.
Rod Pampling sits mid field at T41 while Marc Leishman and Sam Brazel share 52nd position at 4-over for the week and well off the pace.
Scott will be disappointed with his play on day two after continuing on from a poor finish to his opening round.
Holding the lead after 11 holes Thursday, the 2013 Masters champion lost momentum on the back nine and didn’t regain it on day two.
He dropped three shots in his last six holes Thursday and gave up another stroke Friday with a 1-over 71 that sees him six off the pace.
There were no such problems for World Number Four McIlroy who looks in dangerously good form ahead of next week’s PGA Championship.
The 28-year-old put on a clinic with the driver on day two, only one of his 14 attempts with the longest club in the bag not exceeding 300 yards (270 metres).
After splitting with long time caddie J.P Fitzgerald during the week McIlroy is showing no signs of uncertainty in his game as close friend Harry Diamond has picked up looping duties for the short term.
Adding a second round 69 to his opening 67 has the four-time major winner just three off the pace and he will be the favourite for many with 36 holes to play.