Geraint Thomas admitted a brutal schedule had caught up with him as he withdrew from this week's time trial at the UCI Road World Championships in Yorkshire.
The 2018 Tour de France winner will now be replaced by John Archibald in Wednesday's 54 kilometre test against the clock between Northallerton and Harrogate.
Though Thomas will still line up in next Sunday's road race, the Welshman said he did not feel able to deliver his best on Wednesday.
Speaking on his Watts Occurring podcast, Thomas said: "I'm not at the level that I want to be to be competitive. It's not the end of the world but at the end of the day you don't go to the worlds to try and get top 10, or I don't want to anyway.
"You go the worlds to win or be on the podium. Since the Tour I had that break and maybe over-indulged but I still had plenty of time to get into some good shape, but it just hasn't happened.
"Mentally, as much as physically, or maybe more so, I've struggled and I'm not at the right level to do the TT. I'll still do the road race and commit to the boys and the plan there."
Thomas said that since his switch to focusing on targeting the general classification in the Tour in recent years, rather than going to the race in a support role, he has found he feels more fatigue in the second half of the season.
"When you're targeting the GC at the Tour, it's hard to carry on after such a big hit," he said.
And the 33-year-old said he also realised this year how much last winter had taken out of him as he took on a busy schedule following his Tour glory.
Speaking of his post-Tour break this summer, Thomas said: "Suddenly it's like, 'Wow, that was a big 15 months'. When you just pull the pin out and realise how tired you are really, and how stressful it was."
Archibald, who is part of the squad for this Sunday's team time trial mixed relay, will now join reigning national champion Alex Dowsett in Great Britain's two-man squad for Wednesday.
Archibald finished second to Dowsett in June's national championships.
GB's performance director Stephen Park said: "While it's a shame that Geraint is withdrawing from the time trial, we understand and support his decision.
"Geraint will still represent Great Britain in the road race where he will be a real asset to the team.
"John Archibald is ready to ride the time trial in Geraint's place and has shown great form in his preparations for the team time trial mixed relay.
"He's had a good season, finishing second in the elite men's time trial at the national road championships and I have no doubt he will put in a performance that will do the team and the fans proud."
Thomas remains in the six-man GB team for the 285km road race from Leeds to Harrogate on September 29, along with Ben Swift, Owain Doull, Adam Yates, Ian Stannard and Tao Geoghegan Hart.
