The Briton earned a hard-fought victory over Australian Jai Hindley in the final time trial, pushing an average of 430 watts for 18 minutes to end the day 39 seconds ahead of his Sunweb rival.
Geoghegan Hart, who initially entered the Grand Tour working for team leader Geraint Thomas, quickly surpassed expectations with a raft of rides coach Tim Kerrison believed rivalled Bradley Wiggins and Chris Froome from years past.
"There was one day I remember after that stage, the head coach of the team came up to me and he said, ‘the numbers you did yesterday were up there with the best performances we’ve ever had in this team’," Geoghegan Hart told Ben Foster during an interview for the Watford goalkeeper’s YouTube channel.
“I grew up idolising this team. Obviously it’s a bit mad, so when he said that to me, he said, ‘you should be confident because the numbers were up there with all those guys’.
“He might not have even meant it, but the impact it had on me was massive. It just gives you another level of belief in yourself.”
Geoghegan Hart rode around Hertfordshire with Foster using the custom pink Pinarello his team presented to him following his memorable Giro victory.