Titleholder Serena Williams in group with Kvitova, Radwanska, Kerber at WTA Championships

Top-ranked titleholder Serena Williams will face former winner Petra Kvitova in her group at the WTA Championships, which start Tuesday in Istanbul.

Sunday's draw for the event that brings together the top eight players in the world also put Agnieszka Radwanska and Angelique Kerber in Williams' Red Group.

Second-ranked Victoria Azarenka heads the White Group, which also includes Li Na, Sara Errani and Jelena Jankovic.

Williams is looking to become the first player to defend a WTA Championships title since Justine Henin in 2007. If she achieves the feat, she would tie Chris Evert for third most WTA Championships with her fourth.

Kvitova won the event in 2011 and is the only Grand Slam champion in the group apart from Williams.