The victor of the first set had won 24 of the 28 previous matches between the sisters, and Venus made it 25 of 29 on Monday.
But it didn't seem that close.
With each game, with each match, Williams looked stronger, fitter, more technically sound than she had previously. But while there were flashes, she "isn't there yet", as she has reiterated a few times this week.
Wozniacki said the court conditions were hard but she managed to make adjustments.
Serena was a mere shadow of her past self and managed to win just 54 percent of her first serve points in the first set. "It's just that she is out of touch... give her a couple of tournaments and she will be back at her best". "Even though I haven't even played that much this year, the matches in the last year count".
Venus was in the second seat, Serena hanging on the open bench facing in the opposite direction. The other is rejuvenated at 37, back in the world's top 10. Serena returned to the tournament in 2015 and Venus a year later. Federer hit a running forehand passing shot to break at love and take the set 6-2.
"These courts are really hard to play on".
Venus hadn't won a match over Serena since August 2014 at the Rogers Cup in Montreal. Venus had a slight smile; Serena had a sort of a tense-looking smirk.
Venus's victory also comes 17 years after an ugly booing incident led to a 14-year boycott of the tournament by the sisters. They were supposed to meet in a semifinal in 2001, but Venus pulled out due to a knee injury. They sat with their backs to each other, with Serena on the back of the cart with Beats headphones over her ears. "I'm playing a completely different player, new challenges".More news: Ben Stokes pleads not guilty to affray
They were playing first-strike tennis, both going for winners off serve and off return of serve.
Prior to Monday's Indian Wells event, Venus and Serena's most recent contest against one another was last year's Australian Open final. "But if you know how to use disappointment in a good way, it is great motivation".
Venus blasted six aces but had eight double faults in the one hour 26 minutes main stadium match.
For a moment, the best women's player in the history of tennis displayed rare vulnerability.
Up to 7 courts, including the biggest matches of each day.
As with most little sisters, Serena always wanted to win.
The two sisters met on the court for the first time since last year's Australian Open final, when a pregnant Serena won her 23rd Grand Slam title. Her competitiveness is peerless. "My room for improvement is incredible", added Serena.
'So I have just got to keep saying at each tournament that my goal is just to be better than the last. "But I learned she has to move better".