Kopeka and Johnson both struggled as America surrendered the Ryder Cup at Le Golf National in Paris.
And, according to The Sun, they nearly came to blows during the European celebrations.
Witnesses told the Sun Koepka ‘exploded with rage’ and threatened to ‘flatten’ Johnson.
The duo, nicknamed the ‘Bash Brothers’ had popped into the European dressing room for a beer, but things got out of hand and they had to be pulled apart.
The news comes as a shock due to their close friendship off the course.
Europe won 17.5-10.5 after three gruelling days of golf in France.
The Americans struggled to hit their stride after despite leading 3-1 after the first session fell away and never recovered.
Koepka, who won two majors this year, and world No 1 Johnson came into the event as two of America’s biggest hitters but failed to hit their heights.
Koepka recorded 1.5 points while Johnson only took one after losing to European talisman Ian Poulter in the Sunday singles.
Koepka and Johnson was paired together for the Saturday foursomes but lost 2&1 to Justin Rose and Henrik Stenson.
American Koepka was involved in an unfortunate incident when his stray ball on day 1 hit a spectator.
She is considering legal action against the organisation after losing sight in her eye.
The ball went into the crowd and hit Corine Remande, 49, in the eye and left her needing medical attention.
Blood was gushing down her face and she has now said scans have revealed her eyeball exploded and her socket was fractured, causing her to lose sight in that eye.
And she says she will now sue the organisers for failing to keep her safe.