MELBOURNE (Reuters) - Venus Williams's encouraging form at the Australian Open has given younger sister Serena cause to look over her shoulder after the siblings eased into the third round with straight-sets wins at Melbourne Park on Thursday.
For top seed and 18-times grand slam champion Serena, her win over former world number two Vera Zvonareva, on the comeback trail after a 2014 season blighted by injury, was expected.
Venus's struggles with Sjogren's syndrome, an auto-immune disease, have made every match she plays a lottery, but the 18th seed had little trouble accounting for fellow American Lauren Davis 6-2 6-3.
Thirty-four year-old Venus was dumped from the first round last year but now finds herself on a seven-match winning streak after clinching the Auckland Open title in the lead-up.
"She is really motivating. She is playing so well, she's doing so good right now," Serena, who famously completed the 'Serena Slam' by beating her sister in the 2003 final at Melbourne Park, told reporters.
"Yeah, it makes me look behind my shoulders and I have to play better and I want to do better.
"I always want to be able to stay ahead as much as I can. So I think that's been, for our whole careers, we've kind of motivated each other. We hopefully continue to do that."
Venus faces a tough third-round match against 33rd-ranked Camila Giorgi, a feisty Italian counter-puncher, who has broken into the top 50 with a bullet.
Serena, who will meet Ukrainian 26th seed Elina Svitolina, paid tribute to her seven-times grand slam-winning sister for setting up the platform for her own success.
"She came in as a new face, a black woman that was shaking up the world," she said. "She had all the pressure on her shoulders. I kind of came in behind her.
"There was no pressure on me at all... She had a lot of confidence and she had so much class and still does throughout everything.
"You can see that her personality is pretty much the same. She's definitely grown but she's always been very mature and very regal."
Venus said her battles with health problems had not lessened her desire for more titles after more than 20 years as a professional.
"Honestly, I think I understand the game a lot more," she said. "Even when I'm not playing as well, I think I'm able tactically to be more strategic than even, let's say, Venus of 2000.
"So that's one of the beauties of continuing to play as you get a lot of the years under your belt."
Reporting by Ian Ransom; Editing by Patrick Johnston