'To do it for a second time in 2018 feels like a special achievement, especially having also won my first Grand Slam this year.
Though she was on the practice court, as of Monday, her participation in both Moscow and the WTA Finals was still up in the air.
She played in two Grand Slam finals - after losing to Wozniacki in Melbourne she beat Sloane Stephens in Paris - and won WTA titles in Shenzhen and Montreal.
'I'm doing physio to try to make (my back) stronger, but I haven't practised 100 percent yet, ' Halep added.
She has a first-round bye in Moscow, but has yet to decide whether she is fit enough to play.
"Finishing the season as the world No. 1 a year ago was a huge honour for me", Halep said in a WTA news release. "I'm looking forward to meeting the incredible fans in Singapore, exploring the city, and competing against the best players of the season", Stephens was quoted by the WTA website as saying.
Anastasia Pavlyuchenkova will play Simona Halep in the second round of the Kremlin Cup after beating Ajla Tomljanovic in straight sets.
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.
The WTA Finals will see the world's best players vie for a total prize money of US$7 million ($9.65 million) and two of the most prestigious titles in women's tennis. "I give everything I have the first part of the year and then it's hard for me to play", said Halep, who also reached French Open finals in 2014 and 2017 but lost both times.