Australian Open Tennis

Venus Williams of the U.S. celebrates after winning over Agnieszka Radwanska of Poland in their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 26, 2015. (AP Photo/Rob Griffith)

MELBOURNE, Australia (AP) — The Williams sisters are back in the quarterfinals of a Grand Slam, and neither one is there just to watch.

Venus Williams continued her impressive career resurgence at the Australian Open with a 6-3, 2-6, 6-1 win over sixth-seeded Agnieszka Radwanska on Monday night, reaching the last eight at a major for the first time since the 2010 U.S. Open.

Younger sister, Serena Williams, was supporting her and cheering for her from the stands at Rod Laver Arena, just hours after she had to dig deep for a 2-6, 6-3, 6-2 comeback win over Garbine Muguruza on the same court.

The sisters haven’t both advanced this far at a major since Serena won Wimbledon in 2010, their careers seemingly veering off at opposite trajectories.

Australian Open Tennis

Serena Williams of the U.S. celebrates a point won against Garbine Muguruza of Spain during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 26, 2015. (AP Photo/Rob Griffith)

Serena has won five Grand Slam titles in the interim, lifting her tally to 18, while seven-time major winner Venus has labored for years since being diagnosed in 2011 with Sjogren’s syndrome, which causes fatigue and inflammation. She’d only been beyond the third round once at a Grand Slam event since the start of 2011, and many thought she’d never return to the last eight. Even so, she’s not getting too excited four wins into a tournament.

“I guess from the outside looking in, I guess it could look like that,” the seven-time major winner said. “But for me I’m just really focused and poised right now. I feel like I’ve been here before, so it’s not like I’m jumping up and down for joy, ‘Oh, shoot, what is this? I’ve never done this.’

“Yes, I’ve done this. This is what I’m always going into each tournament thinking I want to do. Now is my moment and I want to keep this moment going.”

The 34-year-old Venus next face Madison Keys, a 19-year-old American who was inspired to play tennis by watching the Williams sisters.

Serena was motivated by a Grand Slam loss that stung her more than any last year in her match against No. 24-seeded Muguruza, who had conceded only four games to her in a second-round upset at the 2014 French Open.

“She made me play a lot better,” Serena Williams said. “I had to play the best match of the tournament or else I was going to be out.”

The five-time Australian Open winner next faces Dominika Cibulkova, who reproduced the kind of tennis that took her to the final here last year as she beat two-time champion Victoria Azarenka 6-2, 3-6, 6-3. Keys advanced to her first Grand Slam quarterfinal with a 6-2, 6-4 win over good friend and fellow American Madison Brengle.

“It’s a huge opportunity for me. I’m going to make the most of it,” she said. “I want Io be at the end of the tournament holding the trophy up. That’s my goal in the long run.”

Serena had difficulty breathing at times Monday, and coughed throughout the match, later saying she’d been sick for a couple of days.

There were times when she defended, times when she attacked, and times when she got lucky — like when Muguruza made a mess of an easy volley on breakpoint. But when she needed to assert some authority, she found something special — like her ace, ace, ace, service winner sequence to close the second set, and her crucial hold at the start of the third set when she saved six break points in an almost 13-minute game.

“When I got down, I was thinking, ‘What can I do now?” she said. “Whatever happens, I thought, I’ve won this five times.”

She’ll need every bit of confidence against the 1.61-meter (5-foot-3) Cibulkova, who pounded 44 winners and broke former No. 1-ranked Azarenka’s serve seven times. The No. 10-ranked Cibulkova lost the final here last year to the now-retired Li Na, and is back in that kind of form.

Top-ranked Novak Djokovic had a 6-4, 7-5, 7-5 win over Gilles Muller to reach the quarterfinals at a 23rd consecutive major. Next up, he faces No. 8 Milos Raonic, who moved into the last eight in Australia for the first time with a 6-4, 4-6, 6-3, 6-7 (7), 6-3 win over No. 12 Feliciano Lopez.

Defending champion Stan Wawrinka beat Guillermo Garcia-Lopez 7-6 (2), 6-4, 4-6, 7-6 (8) and will next take on U.S. Open finalist Kei Nishikori.

Spurred on by hundreds of flag-waving Japanese supporters at Rod Laver Arena, No. 5-ranked Nishikori had little trouble in a 6-3, 6-3, 6-3 win over No. 9 David Ferrer, who reached the French Open final and Australian Open semis in 2013.