The 27-year-old Australian moved into contention for the major crown over the weekend at Chambers Bay, despite collapsing in front of the ninth green on Friday due to the illness.
However, the world number 10 failed to go all the way as he shot a four-over 74 on Sunday to finish joint ninth with Rory McIlroy and Shane Lowry, while Jordan Spieth grabbed his second major of the year by winning with a one-shot lead.
"I was taken by the fact that so many people supported me," AP quotes Day as saying. "I really made a lot of Jason Day fans out there this week, even though it didn't end up the way I wanted it to end up.
"Actually, I started feeling a lot better after the 12th hole. I felt relatively good and felt strong. That was a plus. I'm just glad that I got it in on the weekend. Not bad considering."
Day is yet to win a major, but has finished second in the US Open twice - in 2011 and 2013.