Romanian Halep won the Kremlin Cup at the expense of Australian Samantha Stosur last year, triumphing 7-6 (7-1) 6-2 in Moscow and she went on to finish with six titles in 2013.
But the 23-year-old will not return to the Russian capital for the start of the tournament on Monday, with her eyes firmly set on the season-ending WTA Finals in Singapore.
Halep, who withdrew from the China Open at the quarter-final stage due to a hip injury, will remain in Romania until the showpiece event - which pits the top-eight players on the Tour against each other - begins on Friday.
"I talked to therapists of the tournament, which told me that it's better to stop. Going to get better soon and I'll work to have a swift recovery," said Halep.
While Halep joins the likes of Sara Errani, Ana Ivanovic and Angelique Kerber as absentees, Kremlin Cup officials have managed to attract plenty of talent to Russia.
Slovakian Dominika Cibulkova - ranked 12th - is among those set to compete.
Cibulkova won the event back in 2011, when she overcame Kaia Kanepi, though she must improve having not reached a final since April.
Russian second seed and local favourite Ekaterina Makarova heads into the tournament on the back of some decent results.
Makarova beat Eugenie Bouchard and Victoria Azarenka en route to the US Open semi-finals, where she lost to eventual champion Serena Williams.
Since then, the 26-year-old has reached the second round in Wuhan, while she lost to Roberta Vinci in the last 16 at the China Open.
Flavia Pennetta, Lucie Safarova, Svetlana Kuznetsova, Anastasia Pavlyuchenkova, Karolina Pliskova and Carolina Garcia round out the eight seeds in Moscow.