World number one Iga Swiatek, unfazed by two lengthy rain delays, held off Karolina Muchova 6-1, 4-6, 6-4 to reach the quarter-finals of the WTA Montreal Open on Thursday.

Her victory, in a rematch of the French Open final she won two months ago, ensures the Polish star will remain number one in the world for a 72nd straight week when the rankings come out on Monday. It took two hours and 47 minutes on court, but more than nine hours from start to finish on what Swiatek called an "extraordinary" day.