Serena Williams has downplayed concerns over her fitness after struggling through her third-round win over Urzsula Radwanska in the Cincinnati Open.

The American appeared to be suffering with heat exhaustion during the match, but insisted that she will be in good shape for her quarter-final.

"I definitely was a little tired out there today," She told ESPN. "I just thought if I could move my feet a little bit then it could make a big difference. I don't think I ever did it, but I tried.

"At the end of the day, I'm professional and this is my job. Do I want to lose? No. So better toughen up and get through it.

"I was definitely holding back a little bit; struggling a little bit with aches and pains like every athlete, but overall I'm definitely able to manage it. I'll be totally fine."

Williams will face Angelique Kerber, who knocked her sister Venus out of the Olympic tennis, in the quarters.