By John O'Brien SINGAPORE (Reuters) - Angelique Kerber was forced to dig deep in her WTA Finals opener before the German world number one battled past Dominika Cibulkova 7-6(5) 2-6 6-3 in a high-class baseline slugfest at the Singapore Indoor Stadium on Sunday. Earlier, Simona Halep got her campaign off to a perfect start when the Romanian world number four dominated an erratic Madison Keys to kick off round-robin play in the elite eight-woman, season-ending event with a 6-2 6-4 victory. First-time finalist Cibulkova opened with a double fault and Kerber, who won two grand slam titles in 2016, took advantage of her opponent's nervy start to grab an immediate break without having to do much more than get the ball over the net.
Juan Martín del Potro claimed his first title since January 2014 by beating Jack Sock in straight sets to be crowned Stockholm Open champion. Del Potro has endured an injury-ravaged couple of years, but the powerful Argentine has showed great character to demonstrate what he is capable of after returning this season and was rewarded with a long-awaited tournament victory on Sunday. Del Potro took four of the nine break point opportunities that came his way, denying the world number 23 from the United States a second ATP singles title.
Pablo Carreno Busta of Spain came back from a set down to beat Italy's Fabio Fognini and win the Kremlin Cup on Sunday. Carreno Busta won his second career title 4-6, 6-3, 6-2 despite having a medical timeout for a leg strain midway through the second set, though he said the injury was not serious. Fognini saved two match points in the final game before Carreno Busta secured the win, dropping to his knees in celebration.