Torrellis history, "Significant Other" and a few other sources. It was certainly the beginning of a very bad series of events. TdF is a mental game as well as physical and I would imagine that being punched by a spectator while confronting some of the world's greatest climbers could start a decline.
Stage 14 finished on Puy de Dome. Merckx was chasing van Impe and Thevenet up the final climb when a spectator standing in the crowd jumped out and punched the lone Merckx in the stomach. Van Impe won the stage, Thevenet was only 15 seconds behind him. Astonishingly, Merckx was able to finish only 49 seconds behind van Impe and keep his Yellow Jersey. After crossing the line, in agony from the assault, Merckx vomited, then rode down the hill and identified his assailant who said it as an accident. Merckx still had a 58 second lead on Thevenet.

Merckx lost by 2'47s overall.
But "Flaneur", it couldn't have had anything to do with the Frenchman who was merely waving. nahhh thank you for the rightthink