On song: Djokovic, Williams sisters advance at Aussie Open

MELBOURNE, Australia — Novak Djokovic fine-tuned his voice and his game at the Australian Open on Saturday, advancing to the fourth round with a straight-sets win over Fernando Verdasco and then urging the crowd of 15,000 to sing Happy Birthday to his mother back home.

After his 7-6 (8), 6-3, 6-4 win, the four-time Australian champion had a lengthy on-court interview with Jim Courier discussing a range of topics including his childhood memories of tennis, when "I was constructing little trophies out of plastic and pretending I was a Wimbledon champion."

The flashback jogged his memory. It was Jan. 24, his mother Dijana's birthday.

"It's my mom's birthday, can you sing happy birthday?" he asked the crowd, before singing the whole song live on camera.

The Williams sisters had their own celebration of sorts, progressing together to the fourth round of a Grand Slam tournament for the first time since Wimbledon in 2011. Two other Americans joined them, with 19-year-old Madison Keys upsetting two-time Wimbledon champion Petra Kvitova with a 6-4, 7-5 victory and Madison Brengle beating Coco Vandeweghe 6-3, 6-2 in an all-US match. The two Madisons will meet in the fourth round, meaning one will become a first-time major quarterfinalist.

When 18-time Grand Slam champion Serena Williams noticed that older sister Venus was advancing to the second week, it inspired her own 4-6, 6-2, 6-0 win over No. 26-ranked Elina Svitolina.

When top-ranked Serena walked onto Rod Laver Arena, Venus was already down a set and a break on Margaret Court Arena. Venus, who was diagnosed with an auto-immune condition called Sjogren's syndrome in 2011 and has struggled at the highest level ever since, rallied for a 4-6, 7-6 (3), 6-1 win over Camila Giorgi, and that inspired her younger sister.

"I thought, 'Wow, she's been through so much with her illness, with everything that she's had to do. Gosh, if she can do it, I'm perfectly healthy, I'm fine. I should be able to do it, too," the five-time Australian Open champion said. "It just got me so motivated, really helped me push through those next two sets at a rapid rate."

She'll have to be at the top of her game in the next round she meets No. 24 Garbine Muguruza, who beat her in the second round at the French Open last year. Muguruza defeated Timea Bacsinszky 6-3, 4-6, 6-0.

In the other fourth-round match in that quarter, two-time champion Victoria Azarenka will meet 2014 finalist Dominika Cibulkova.

Azarenka, the winner here in 2012 and 2013 but who is unseeded this year because of an injury-interrupted 2014, had a 6-4, 6-4 win over No. 25 Barbora Zahlavova Strycova. Cibulkova beat No. 19 Alize Cornet 7-5, 6-2.

On the men's side, defending champion Stan Wawrinka had a 6-4, 6-2, 6-4 win over Jarkko Nieminen and will have a score to settle in the next round against Guillermo Garcia-Lopez, who beat Vasek Pospisil in straight sets.

Wawrinka lost to Garcia-Lopez in the first round of the French Open last year, making him the first first-time major winner in 12 years to lose in the first round of his next Grand Slam event.

No. 8 Milos Raonic fired 22 aces in a 6-4, 6-3, 6-3 win over Benjamin Becker and will next play No. 12 Feliciano Lopez, who had a 7-6 (6), 6-4, 7-6 (3) win over Jerzy Janowicz.

No. 5 Kei Nishikori, the U.S. Open finalist, beat Steve Johnson 6-7 (7), 6-1, 6-2, 6-3 and will next meet former French Open finalist David Ferrer, who struggled to finish off No. 18 Gilles Simon in four sets.

Gilles Muller beat No. 19 John Isner 7-6 (4), 7-6 (6), 6-4 to set up a fourth-round clash with Djokovic.

Venus Williams couldn't contain her delight after her win, beaming a smile as she turned to wave to all sides of the stadium. She said the sibling relationship, and rivalry, helped both of the Williams sisters.

"I've been motivated by Serena though since day one. The way she faces her life, the way she is fearless on the court," Venus Williams said. "We've had the fortunate relationship to be able to motivate each other and grow from each other."

The 34-year-old, seven-time major winner next faces sixth-seeded Agnieszka Radwanska, who advanced with a 6-0, 7-5 win over Varvara Lepchenko.

"I like to win titles ... that's what I play for," she said. "So, yes, it's great to be in the second week but is fourth round my goal when I come to these tournaments? No."