Stephens, 25, won her first Grand Slam title at Flushing Meadows in 2017, but was knocked out in the quarter-finals by Anastasija Sevastova this year.
A week away from the fifth and final WTA Finals in Singapore (it's moving to Shenzhen, China), Halep will be the only player to have appeared in all five editions.
Stephens has earned her career first qualification at the WTA Finals following a stellar season, which saw her win her first WTA Premier Mandatory title at the Miami Open.
'I'm doing physio to try to make (my back) stronger, but I haven't practised 100 percent yet, ' Halep added.
This means that Elina Svitolina, Karolina Pliskova and Kiki Bertens will decide the remaining two travelers to Singapore but it has to be said that Svitolina made a decision to skip Moscow, leaving the door open for both of her rivals to pass her and plague her chances of reaching the WTA Finals.
World number eight Sloane Stephens has qualified for the season-ending WTA Finals for the first time.
"I had many attempts to become No 1 and it was really tough mentally to keep believing that it would happen one day, but then it was really tough to stay there because I felt the pressure at every tournament", she said.
Apart from a four-week spell at the start of this year when she was briefly overhauled by Caroline Wozniacki, Halep has kept hold of the world No 1 ranking ever since she went top early in October last year. "I'm looking forward to meeting the fantastic fans in Singapore, exploring the city, and competing against the best players of the season".
Anastasia Pavlyuchenkova will play Simona Halep in the second round of the Kremlin Cup after beating Ajla Tomljanovic in straight sets.
Both Pliskova and Bertens could move in front of Svitolina with good runs in Moscow, but she will qualify if either does not.
Dealing with a herniated disc, Halep is in Moscow this week to test out her back and get some match prep before Singapore.