The Americans are both sitting at 5-under par after two rounds - three shots back of clubhouse leader J.B. Holmes heading into the weekend.
After opening with a 1-under 70, Spieth used an electric four-hole stretch on the front nine on Friday to launch into contention. The 2017 Open champion made three birdies and an eagle from the fifth through the eighth and finished with a 4-under 67. Spieth has slumped this season, but a third-place showing at the PGA Championship in May - along with the strong start this weekend - has shown he can still play with the world's best.
Meanwhile, there's no debate about Koepka's place among the game's elite, as the top-ranked player in the world is yet again poised to contend in a major. The 29-year-old produced a solid round on Friday, mixing in three birdies against one bogey to finish with a 69. That leaves Koepka well-equipped to contend for a fifth major title in his last 10 starts.