Rafael Nadal's hopes of a first ATP Finals title took a blow as he was beaten 6-3 6-4 by Felix Auger-Aliassime.
The 36-year-old Spaniard must win the year-end title to finish 2022 as world number one.
Victory against Norway's Casper Ruud in his final Green Group match may not be enough to keep his hopes alive.
Ruud, who has already beaten Auger-Aliassime, needs to just win a set against Fritz later on Tuesday to eliminate Nadal.
An early exit for Nadal would mean his 19-year-old compatriot Carlos Alcaraz, who won the US Open in September, would end the year as world number one.