Serena Williams, coming off a double Olympic gold medal-winning performance in London, 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 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.
"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 U.S. Open champion Samantha Stosur.
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.