All the women's tennis storylines of 2015 talked about potential candidates that could challenge Serena Williams as the game's best player.

It appears, one challenger may have been overlooked - or people were worried about the wrong Williams.

'Energized' Serena Williams taken to third-set tiebreak against Flavia Pennetta

Venus Williams, Serena's older sister, started off the New Year with a bang, upsetting top-seeded Caroline Wozniacki 2-6, 6-3, 6-3 to with the ASB Classic on Friday. At age 34, Venus became the fourth-oldest winner ever on the WTA Tour, according to The Guardian.

Only Billie Jean King, Kimiko Date-Krumm and Martina Navratilova won WTA events when they were older.

Maria Sharapova offers unusual compliment to rival Serena Williams

"I am an old tennis player now," Williams said. "I waited until I was 33 until I came to New Zealand and that was the biggest mistakes of my life."

She lost in the final of last year's ASB Classic to Ana Ivanovic.

The seven-time Grand Slam champion won for the first time since February of 2014 in Dubai - in which she defeated Wozniacki in the semifinals - and captured her 46th career title overall. Victories have been much harder to come by for the elder Williams ever since she was diagnosed with Sjogren's syndrome, a debilitating autoimmune disease, in 2011.

Serena's sister has not won a Grand Slam since the 2008 Wimbledon championship.

She told CNN that a new vegan diet and exercise regimen has helped her recoup some of the energy that the disease had taken from her.

Her win in Auckland makes Venus a dark horse at next week's Australian Open - because it is a two-week event, the odds become more stacked against Williams enduring through the tournament.

But she has had the more impressive week than sister and No. 1-ranked Serena. Williams suffered a 6-2, 6-1, 50-minute route at the hands of Eugenie Bouchard at the Hopman Cup to start her week. She then struggled to three-set victories over Flavia Pennetta and Lucie Safarova.

On Saturday, Serena lost to Agnieszka Radwanska 6-4, 6-7 (3), 6-1, Radwanska's first-ever win in nine matches against Williams. She then lost her temper, ESPN reported, in her mixed doubles loss alongside John Isner against Radwanska and Jerzy Janowicz as Poland beat the U.S. in the final of the Hopman Cup.

During the first two matches, Serena herself complained of fatigue, and though she told the Guardian that she felt better against Safarova, that match went to a third-set tiebreaker in which Serena won 8-6 in the tiebreak. She also was reported to be at full strength against Radwanska.

So if a Williams is a favorite at next week's Australian Open, it's unclear which one it will be.