Didn't see it myself but the scoreline tells the story. Nishikori took the first two sets but was then reminded that beating top 10 players is never a cake walk.
Here's Nick Bolletieri's summary:
Nishikori won the match 6-4, 6-4, 3-6, 2-6, 7-5. It seemed like every time Ferrer broke Kei in key spots, Kei was able to break back to get the match back on track. Kei is now the first Japanese male in the Open Era (since 1968) to reach the fourth round of the US Open, and is the second Japanese man to ever reach the fourth round of a Grand Slam. The last Japanese men to reach the Round of 16 here were Fumituru Nakano and Jiro Yamagishi in 1937. He is also the youngest male to get this far since Marat Safin in 1998, and the is youngest man to defeat a Top 4 seed here since 17-year-old Bjorn Borg defeated No. 3 seed Arthur Ashe in 1973 3rd round.
Looking forward to watching him against Del Potro, one of the other up and coming teenagers, later today. Also hoping to catch some of Nadal v Querrey.
You can watch Kei close it out below