Johnson was at 7 under par through 14 holes, only to bogey Nos. 15 and 18 to finish at 5 under, five back of leader Phil Mickelson. He shot 1-under 70.
Despite the deficit – and the finish – he remains optimistic.
“Yeah, absolutely. I still feel like my best game is in front of me. I feel confident in the sense that I'm doing the right things,” he said.
Johnson said his game is in great shape and that he has nothing to fix; he just needs to sharpen some areas.
Johnson is looking to become the first player since Padraig Harrington (2007, ’08) to win back-to-back claret jugs. Tiger Woods (2005, ’06) is the only other player in the last 32 years to successfully defend his Open title.