This article may contain graphic and/or adult content unsuitable for minors and sensitive readers.
Victoria Azarenka had to draw on all her reserves by saving two match points in a heroic three-hour tussle with Angelique Kerber in the WTA Championships in Istanbul.
The top-seeded Belarussian beat the German 6-7 7-6 6-4 and now needs just one more win to guarantee her finishing the year as world number one.
Maria Sharapova also made sure of a place in Saturday's semi-finals by beating Agnieszka Radwanska 5-7, 7-5, 7-5 in another three-hour encounter.
Serena Williams reached the last four with a straight sets win over Li Na of China.
