Cameron Norrie will make his debut at the Nitto ATP Finals on Wednesday night after Stefanos Tsitsipas withdrew through injury.
British No 1 Norrie travelled to the prestigious tournament as second alternate after narrowly missing out on qualifying for the eight-man event.
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With first alternate Jannik Sinner having already replaced fellow Italian Matteo Berrettini, Norrie has been given an unexpected chance.
He becomes the first British singles player to appear at the tournament since Andy Murray won the title five years ago, and will take on Norway's Casper Ruud on Wednesday evening, before meeting Novak Djokovic on Friday.
To stand a chance of qualifying for the semi-finals, Norrie would need to win both matches and hope other results go his way but, with 200 ranking points and more than £120,000 at stake for each group contest, there is plenty to play for.
Tsitsipas was a doubt for the tournament after withdrawing from the Paris Masters two weeks ago with a right elbow problem, and he declined to answer a question on his fitness after losing his opening match to Andrey Rublev.
This is the first time since 1998 and only the third time in the tournament's history that two alternates have competed, and all three have involved British players. Tim Henman gained entry in 1997, with Greg Rusedski following suit the next year.
Djokovic, who is chasing a record-equalling sixth title, booked his place in the semi-finals with a 6-3 6-2 victory over Rublev.
As he had in his opening match against Ruud, Djokovic was broken in his opening service game, but from there it was smooth sailing for the world No 1.
Jamie Murray and Bruno Soares' campaign looks set to end in the group stages after they suffered a second straight defeat in Turin.
After losing to Britain's Joe Salisbury and American Rajeev Ram on Monday, Murray and Brazilian Soares were beaten 6-2 6-4 by Colombian duo Juan Sebastian Cabal and Robert Farah.
Murray and Soares are not yet out of contention for the semi-finals, but it would take a specific series of results over the last three matches in the group to give them any chance.
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