Angelique Kerber has made it into the final four of the WTA Finals with a 100% record in the red group in Singapore.
The world number one secured her spot in the semi-finals with a 6-3 6-3 triumph over Madison Keys.
Both players showed fighting spirit by earning double breaks, but Kerber took the edge by converting her second set point to take the lead in 29 minutes.
Keys sparked a comeback when she snatched serve in the first game of the second set, but Kerber regained composure to break twice before wrapping up the match.
As a result of the straight-sets win, Dominika Cibulkova has also progressed into the semi-finals following victory over Simona Halep.
