Unwell Serena Williams into Cincinnati Open quarter-finals

Serena Williams has seen off Urzsula Radwanska in straight sets to reach the quarter-finals of the Cincinnati Open.

Despite dropping her serve twice, the second seed won an unpredictable first set 6-4.

The Olympic gold medallist looked to be suffering from an illness in the second as Radwanska took a 3-1 lead in the Ohio heat.

However, the American broke back three times to secure her passage into the last eight 6-3.

Williams will now play German fifth seed Angelique Kerber for a semi-final spot.