After a slump at the start of the year, Sloane Stephens managed to change her fortunes and consolidate her position in the top-10 of the rankings with several performances, and her breakthrough on clay which saw her reach the final of Roland Garros.
She also won the Shenzhen Open and Canadian Open titles this season and made it to the final round at the Australian Open, Italian Open and Cincinnati Masters.
"Finishing the season as the world number one a year ago was a huge honour for me", Halep said in a WTA news release.
'I'm doing physio to try to make (my back) stronger, but I haven't practised 100 percent yet, ' Halep added.
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Stephens joins Romania's Simona Halep, Germany's Angelique Kerber, Denmark's Caroline Wozniacki, Japan's Naomi Osaka and the Czech Republic's Petra Kvitova who have already secured a spot in the tournament.
"Finishing the season as the WTA world number one previous year was a huge honour for me", said Halep. "If I can't play here I'm very doubtful that I will be able to play in Singapore because it's so soon". I don't know now, but for sure I will take a decision for my health first'.
Halep has played in 15 tournaments so far in 2018, but she followed up her first major title by losing in the third round at Wimbledon.
Hence, as a result, Stephens has managed to qualify for her maiden WTA Finals appearance through a series of excellent results. In her attempt to defend her US Open title, she produced a spirited run to the last eight having defeated Mertens and Azarenka along the way.