Wales' understudies failed to deliver a World Cup boost as they slipped to a 2-1 Nations League defeat to Poland.
Just four days before their World Cup play-off final, Jonny Williams' second international goal gave an unfamiliar Wales line-up a 52nd-minute lead in Wroclaw.
But Poland substitutes Jakub Kaminski and Karol Swiderski struck in the final 18 minutes to turn the game on its head and inflict a first defeat on Wales since their Euro 2020 loss to Denmark last summer.
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Captain Gareth Bale was absent from Wales' matchday squad after arriving from Real Madrid's Champions League celebrations, with boss Robert Page shuffling his pack ahead of Sunday's decider in Cardiff.
Bale had become the first British player to win the Champions League on five occasions when Real beat Liverpool on Saturday, and Page refused to risk his talismanic forward for the Nations League opener.
Aaron Ramsey, Ben Davies and Joe Allen were also rested as Ipswich wing-back Wes Burns won his first cap.
Burns was in Wales' preliminary squad for Euro 2016 before missing out on the tournament in France.
This was Wales' first fixture in the top tier of the Nations League following their promotion two years ago, but Page's team selection was understandable given the magnitude of Sunday's game.
Poland had secured their own World Cup qualification in March, and skipper Robert Lewandowski was the centre of attention following his promise to quit Bayern Munich this summer.
Jan Bednarek and Mateusz Klich were representing the Premier League, but Aston Villa defender Matty Cash missed out through injury.
Wales fielded an unfamiliar back-three of Chris Gunter, Chris Mepham and Rhys Norrington-Davies and they should have fallen behind in the first minute.
But the unmarked Piotr Zielinski sent his header wide of the near post and Mepham soon advanced quickly to block Lewandowski's powerful shot.
Lewandowski had a clearer sight of goal after 21 minutes when two Wales defenders collided, but Danny Ward reacted well to push his effort around a post.
Dylan Levitt saw an early effort held at the second attempt by Kamil Grabara, and Wales carried a genuine counter-attacking threat through Daniel James and Kieffer Moore.
Their intelligent combination play stretched the Poland defence and Moore was off target following one incisive break.
James failed to take Wales' best opening after being found superbly by Joe Morrell, the Leeds forward sliding his shot wide of Grabara's near post.
Poland were also guilty of wastefulness in front of goal as Adam Buksa failed to turn in Tymoteusz Puchacz's inviting cross.
Wales withdrew James and Moore at the interval with Mark Harris and Rabbi Matondo coming on and Wayne Hennessey replacing Ward.
The move paid off as Wales took a 52nd-minute lead when Williams picked up possession on the edge of the Poland area and arrowed a low shot past Grabara.
The rhythm of the game was interrupted by a raft of substitutions and Poland struggled to respond to falling behind.
But Kaminski was afforded time and space in the Wales penalty area to beat Hennessey from 12 yards.
It got better for Poland five minutes from time after some pinball in the Wales box.
Lewandowski's shot was diverted into the path of Swiderski, and the substitute bundled the ball home from close range.
Wales boss Robert Page admitted afterwards that his team selection was influenced by Sunday's World Cup play-off final, which will be against Ukraine.
"I didn’t leave anybody out because there were injury concerns," Page said. "I left them out purely because we’ve got the game on Sunday.
"There was a group that trained today and I already had the substitutions planned before the game.
"The plan was already in place. I didn’t need to risk Aaron, Gareth, Ben, Joe, there was no benefit in that whatsoever.
"They’ve done a controlled session, what they need, to get them ready for Sunday.
"I made sure they were in the changing room. They’re a massive influence on that group and I wanted them in there.
"Even at half-time I wanted them to be a part of it and they were the first ones through the door at the end."