Jordan Spieth breathed new life into his bid to complete the career grand slam at the US PGA Championship, although defending champion Brooks Koepka clearly had other ideas at Bethpage.
Spieth needs to win the US PGA to join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods in having claimed all four major titles but was rated a 50-1 outsider to achieve the feat this week after enduring a poor season.
The former world number one looked good value at that price after adding a second round of 66 to his opening 69 to set the early clubhouse target on five under par, a shot ahead of world number one Dustin Johnson and compatriot Daniel Berger.
But overnight leader Koepka, who equalled the lowest score in tournament history with his opening 63, looked in no mood to relinquish his crown and claim a remarkable fourth major title in his last eight starts.
Koepka birdied three of the first four holes on his way to a front nine of 32 to reach 10 under par, five shots clear of Spieth and England's Tommy Fleetwood, who was runner-up to Koepka in last year's US Open and fourth behind the American in the same event in 2017.
Masters champion Tiger Woods carded two birdies and two bogeys on the front nine to remain two over par and one shot inside the projected cut line, with Rory McIlroy right on the mark after a battling 71.
McIlroy had limped to the turn in 40 after starting his round double bogey, bogey, double bogey, but the four-time major winner produced a hat-trick of birdies from the fourth and picked up another shot on the eighth.
"I just needed to see one putt to go in, to see something hit the bottom of the hole and that was on the fourth," McIlroy said. "From there I started to play some good golf.
"I had a horrendous start, five over after three, but came back well and the goal after those three holes was to be here for the weekend and it looks like I have done that, which is nice."
Spieth was second in the world after winning the third leg of the grand slam in the 2017 Open Championship but has not tasted victory since and is currently ranked 39th after failing to register a single top-20 finish this season.
The 25-year-old admitted at the start of the week he was "in a bit of a slump" but carded six birdies and two bogeys to start a major with two rounds in the 60s for the fourth time. He won the other three.
Asked if the prospect of joining golf's most elusive club had crept into his mind, Spieth said: "It certainly hasn't. I can't imagine it will because I haven't been in contention on a Sunday since the Masters last year.
"If I'm able to put some good work in tomorrow, then I will be in contention on Sunday and at that point, it will be just more (thinking) of trying to win a golf tournament. It won't matter to me what tournament it is."
The best round of the day so far had come from England's Matt Fitzpatrick, whose 65 was a 10-shot improvement on his opening round and left him level par overall.
"I feel like the tougher courses suit my game a little bit more," Fitzpatrick said.
"You can't get away with anything 40 yards offline. I think the furthest offline I've hit this week is 10 yards from the edge of the fairway. I've hit a lot of fairways so this has been the key."
