Venus Williams won 6-3, 6-4 against little sister Serena in the third round of the BNP Paribas Open on Monday night.
This is the earliest the pair will meet in a tournament since their 2nd round match in the 1998 Australian Open.
"It's a huge difference to play her in the semifinals or even the quarterfinals or a final as opposed to the third round", she said.
"I was struggling finding my rhythm", said Wozniacki, who quickly went down 0-3 in the first set.
However, she revealed in February that she "almost died" after giving birth and was bedridden for six weeks.
Serena always calls her sister her toughest opponent - even when she's going full throttle. Serena ended the match with a forehand failing to find the court, one of her 41 unforced errors.
"She (Serena) kept roaring back, I'm just lucky that I've played more matches than her right now".
The recovering Serena expressed disappointment over the result despite just gradually making a return to the court.
Elsewhere, Australian Open champ Caroline Wozniacki benefited from a challenge call in the final game en route to defeating Aliaksandra Sasnovich 6-4, 2-6, 6-3. She emphatically closed out the 36-minute set with back-to-back aces that clocked 108 miles per hour and 120 miles per hour. She has won both her matches in straight sets. But Serena cracked a backhand victor down the line for deuce.
Atsushi Tomura via Getty Images Venus Williams during her women's singles first round match against Mona Barthel of Germany during day one of the Toray Pan Pacific Open at Ariake Colosseum on 22 September 2013 in Tokyo, Japan.
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Serena held to trail 5-4 with an ace and a clean victor. Venus faced three deuces, saved break point with an ace, and finally held. Venus hung on from there to end their 29th meeting. She lamented missing shots she said she would make 10 times out of 10 in practice. "At least they're in the margin", Serena said.
Venus was leading 5-2 when she wasted a match point and the 37-year-old needed to fend off a break point in the 10th game before closing it out. "I really already was resetting, because the tournament is far from over".
It was obvious both were not over the moon, as neither smiled when they greeted after the game. Venus did her usual victory pirouette, but still didn't smile. My room for improvement is incredible.
Unseeded American Danielle Collins continued her unlikely run with a 6-4, 6-4 win over Sofya Zhuk of Russian Federation.
Her stamina appeared to get better as the match progressed, and she won several long points.
The sisters were supposed to play each other in the desert in 2001. It felt this close to a Serena breakthrough moment at times.
Serena went on to play Kim Clijsters in the final.
Obviously that led to a few complaints from the fans on Twitter.
Richard Williams said he heard racial slurs from the crowd and he paused to shake his fist at them.
Monday night was supposed to be all about Serena Williams.