Venus Williams fought off her sister Serena 7-6 (7-5) 6-2 to reach the quarter-finals of the US Open.
Serena came into the match in shaky form but she looked sharp in the opening exchanges.
Venus stepped up a gear to move to 5-3 but a series of unforced errors allowed her sister to get back to 6-6.
The tie-break was hard-fought but Venus edged it and her accuracy from the back of the court proved too much for her tiring sister in the second set.
Venus will face in-form Kim Clijsters in the last eight after the Belgian thrashed Maria Vento-Kabchi.
The Wimbledon champion said she had not enjoyed the experience of playing against her sister.
"I think it was distracting for both of us to be honest," she said.
"I feel like I had some bad patches, Serena had some tough patches, too, and then some good ones. It was tough."
The Wimbledon champion added: "I'm dedicated to getting my ranking up. I feel like, wow, I'm a very good player. I know I'm better than number 10."
Serena arrived at Flushing Meadows having played just one match since Wimbledon because of knee and ankle injuries and Venus said she sensed her sister was not moving at her best.
"Serena is so tough she'll never admit to anything," she said. "To me it looked like it. I was a lucky girl."
Serena later confirmed: "I was having problems at the end because I was moving a lot and stopping a lot.
"It always gives me a little trouble after a certain time period."
But the former world number one insisted she is close to regaining her best form.
"I don't think there's anyone besides Venus that could have beaten me here," she said.
"I'm playing better than I did when I won the Australian Open in January and I'm excited I have that confidence again."
Serena beat Venus in five Grand Slam finals in 2002 and 2003 but Venus has now won their last two encounters.