"It is heart-breaking for me to not have a chance to play and help Belarus in the final but unfortunately, my current custody situation is keeping me in California," the 28-year-old said in a statement posted on her Twitter account.
"Even though I will not be in Minsk for the final my heart is always with my country and my team and I will cheer for them from afar."
The two-times grand slam winner returned to action in June after the birth of her son Leo last December and decided to skip the U.S. Open in September amid the custody battle.
Belarus, who will host the Nov. 11-12 decider, have never won a Fed Cup championship while the United States won the last of their record 17 titles in 2000.
(Reporting by Frank Pingue in Toronto; Editing by Peter Rutherford)
