McFly star Judd was the champion 10 years ago and 2021 contestant Fletcher, who recalled coming down to the Strictly studio for every live show but one to support his friend, is still inspired by the effort he saw Judd make.
Singer Fletcher, who is competing with Amy Dowden, told Yahoo UK: "I loved watching Harry, he's my best mate and he'd never danced before and we all went down there taking the mickey out of him and then he came on and was amazing.
"Actually, he wasn't amazing straight away but you could just tell he had potential to do well and he worked so hard and that was really inspiring."
Dad-of-three Fletcher admitted he still looked to his friend for tips on how to approach the show.
He said: "We went back and watched a load of Harry's dances and if you watch him in the first couple of weeks to then when he gets to the semi-final and the final, he's like a completely different dancer.
"That was inspirational to me to know that wherever I am in week one if I work hard, I'll be able to get to where I am."
The children's author also recalled watching Judd's win with Aliona Vilani and storming the stage as his name was called out as the champion: "We ran on, so I'd already been on the Strictly dancefloor.
"The only week I wasn't there was when I cheated on Strictly and went over to Australia because Dougie (Poynter) was in the I'm A Celebrity final."
Fletcher admitted he has had his toughest week so far in the contest learning the Viennese Waltz, but was keeping his fingers crossed that he would make it through to his favourite episode next week.
He said: "I really wanted to get to musicals week which is next week. I went to theatre school growing up and was in musicals from eight or nine, and my sister is a musical theatre actress so musicals are a big part of my family's life...I'd be gutted to go home this week."
Fletcher and his wife Giovanna have both recently published new books and the singer said he felt lucky that life was so busy: "I play the O2 with the band next week so that'll be a busy one.
"I don't know, on paper it doesn't work (to do everything), but you just get your head down. And also it's fun, I get to be in a band, how often do you get to learn to dance from Amy Dowden, these are amazing things and writing books, I love everything that I get to do. I love working hard because it's all fun."
Strictly Come Dancing continues on Saturdays and Sundays on BBC One.
Watch: Who is Strictly's Tom Fletcher?