Brooks Koepka was in one of the early groups out on the course at Bethpage Black in the 101st PGA Championship, so his course-record 63 lingered over the rest of the field as everyone tried to chase the defending champion and clubhouse leader's early advantage at 7 under. By the end of the afternoon only one player, Danny Lee, was within three strokes of Koepka. 

Dustin Johnson, Jason Day, Jordan Spieth and Phil Mickelson are all in the top 10 on the leaderboard technically, but that top 10 doesn't feel as close to the top when the leader is up six strokes heading into Friday. 

The largest comeback to win we've seen in PGA Championship after 18 holes was eight strokes, so in theory, there are 52 golfers still within that comeback precedent. Tiger Woods, at 2 over, is not included in that group. Woods started his day with a double bogey and spent much of the early round grinding with few birdie opportunities. When he got good looks, he was able to take advantage and found himself moving with some momentum after back-to-back birdies. 

With just 17 players under par, a big winner on Thursday was Bethpage Black itself. Koepka and Lee may have come out unscathed, but it's unsustainable to expect those same results the rest of the week. When the setbacks come, the leaders are going to be required to have a championship caliber response in order to keep this cushion on the rest of the field.