Angelique Kerber has pulled out of the Kremlin Cup, citing a back injury as the reason for her withdrawal.
The German needed a medical time out in the final set of her 3-6 7-6 (7-4) 6-1 defeat to Jelena Jankovic in the final of the Hong Kong Open on Sunday.
The WTA subsequently confirmed via Twitter that a "non-acute back injury" will prevent the German from competing in Moscow this week.
Kerber would have secured a place in the season-ending WTA Finals in Singapore by winning the title in Hong Kong.
She remains well placed to qualify and is sixth in the Road to Singapore standings.
Kerber joins the likes of Petra Kvitova and Agnieszka Radwanska - who have qualified for the Finals - in pulling out of the Kremlin Cup.