Saint Petersburg - Venus Williams blamed poor fitness for her 55-minute defeat to Kristina Mladenovic at the St Petersburg Trophy on Thursday, five days after losing to sister Serena in the Australian Open final.
The 36-year-old, seeded four, slumped to a 6-3, 6-1 defeat to her French opponent in her second round tie after receiving a first round bye.
"Today I wasn't in my best of health," Williams said.
"I felt pain in my legs. I had similar feelings during my last match in Australia. I also felt some lack of energy. Definitely it wasn't the best match in my life.
"Also she (Mladenovic) played really well. She has played almost a perfect match and gave me no chance today."
Mladenovic raced into a 5-0 lead before Williams, who was experiencing problems with her left thigh, chalked up her first game of the match.
After taking the opening set in 28 minutes Mladenovic, the world number 51, broke at the start of the second.
She produced two more breaks to secure her win and to set up a quarter-final meeting with defending champion Roberta Vinci of Italy, who prevailed 6-4, 6-4 over German qualifier Andrea Petkovic.
"It wasn't as easy as the scoreline indicates," Mladenovic said.
"Venus is a top player and a great champion. For sure it was difficult for her to come here and compete at her best after playing in Australian Open final.
"I tried to play very consistent and aggressive (tennis) from the beginning, trying to give her no time to adjust to the new conditions and it paid off.
"I have a lot of respect for Venus as I grew up watching her playing. And I'm proud to play with her on the same court and win."
Vinci, the 33-year-old Italian and seeded sixth, needed an hour and 37 minutes to record her second win over the 52nd-ranked Petkovic in their third meeting.
"It was a difficult match," Vinci said. "I started well and led 4-0 in the first but Andrea found a way to come back."
Second seed Dominika Cibulkova of Slovakia, winner of last year's WTA finals in Singapore, experienced few troubles on her way to the last eight beating Croatian lucky loser Donna Vekic 6-2, 6-2.
In the quarter-finals, Cibulkova will meet fifth seed and local crowd favourite Elena Vesnina, who battled back from a set down to beat Alize Cornet of France 2-6, 6-3, 6-3.
"I just couldn't lose because my coach (former tennis star Andrei Chesnokov) celebrates his 50th birthday today," Vesnina said.
In Friday's other quarter-finals, top seed Simona Halep will face Russian wildcard Natalia Vikhlyantseva while third seeded Svetlana Kuznetsova tackles Yulia Putintseva of Kazakhstan.