Fatigued Serena labours into Cincinnati quarters

Serena Williams [ Images ], coming off a double Olympic [ Images ] gold medal-winning performance in London [ Images ], said she was feeling the pace of a hectic schedule after a lethargic display in her 6-3, 6-0 win over Urszula Radwanska on Thursday.

Williams made a series of unforced errors and showed little movement as she laboured her way through a 70-minute match in humid conditions to earn a spot in the quarter-finals at the Cincinnati Open.

"I was out there and I did the best I could today. I was happy to get through that match," said Williams, who won six consecutive games to finish off the match.

"It was just energy. I have been playing a lot, been travelling a lot, been training a lot. It's been a really big summer. Singles and doubles is a tough schedule."

Since the start of Wimbledon [ Images ] in June, Williams has played 30 matches, with 19 in singles play and 11 in doubles.

But Williams expects to be in better shape for her quarter-final matchup against Wimbledon semi-finalist Angelique Kerber.

"I think it's just a one-day thing. I will get some rest," said Williams, who retained the Olympic tennis women's doubles gold with her older sister Venus in London.

The 14-times Grand Slam winner did spare a thought for her vanquished opponent Radwanska, who is the younger sister of Polish world number three Agnieszka Radwanska [ Images ].

"They're both great players, great sisters. I absolutely love seeing another pair of sisters in the locker room that's doing so well," said Williams.

"I think Urszula has worked really hard, and me being a younger sister and Urszula being a younger sister, I'm always looking for her to do well and hoping she does really well."

Sister Venus also booked her place in the last eight with a routine 6-3 6-0 dispatching of Italian Sara Errani which earns her a clash with US Open [ Images ] champion Samantha Stosur [ Images ].

Australian Stosur dealt with Russian Ekaterina Makarova 6-1, 6-3 while Agnieszka Radwanska needed three sets to deal with determined American Sloane Stephens 6-1, 4-6, 6-4.

Photograph: Nick Laham/Getty Images