The top-ranked Williams was pushed all the way by No. 24-seeded Muguruza, who matched her in the heavy-hitting stakes, and who had conceded only four games to her in a second-round upset last year at the French Open.

Five-time Australian Open winner Williams 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.

Defending champion Stan Wawrinka beat Guillermo Garcia-Lopez 7-6 (2), 6-4, 4-6, 7-6 (8) to move into the men's quarterfinals, where he'll play 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.

His routine win was unlike the three-setter Williams endured, when she appeared to be laboring and had a serious cough. Williams struggled with her serve in the first set, winning only one of a dozen points on her second serve. She couldn't convert her six break-point chances, while Muguruza broke her twice.

"She hits the ball really, really big, really, really hard," Williams said. "Someone in the crowd was like 'C'mon Serena use spin' and I was like 'OK.' There's coaches everywhere out here! Thank you."

Madison Keys advanced to her first Grand Slam quarterfinal with a 6-2, 6-4 win over good friend and fellow American Madison Brengle.

Williams hasn't advanced past the quarterfinals at Melbourne Park since winning her last title here in 2010. She went out in the fourth round last year, and followed that up with the 6-2, 6-2 loss to Muguruza at Roland Garros '€” her most lopsided loss at a major.

"She made me play a lot better. I had to play the best match of the tournament or else I was going to be out," Williams said of her avenging win. "I think she was just hitting winners like left and right. Every shot I hit, she basically hit a winner on. So I had to change my approach."

There was a lot to distract Williams in the first set. Six planes were doing coordinated loops and tricks in the sky over Rod Laver Arena, as part of the national day celebrations in downtown Melbourne. She had difficulty breathing at times, and had a heaving coughing fit into a towel at a changeover.

"I've been a little sick the past couple days but I just love it here," Williams said, when asked if she'd swallowed a fly. "When I got down, I was thinking, "What can I do now? Whatever happens, I thought, I've won this five times."

She gradually picked up power in the second set, broke for a 5-3 lead and then fired three aces and a service winner to level the match at 1-1, screaming as she pumped her fist.

After missing break points on Muguruza's serve in the opening game of the third, Williams saved six break points and needed almost 13 minutes to hold.

Her confidence up, Williams broke for a 3-2 lead and dominated the rest of the match.

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.

"I just walked onto the court and all great memories came to my mind," she said. "I thought 'I'm a great player and I can do it. As you can see, I'm not the tallest player on tour, I need to have something extra if I want to beat these top players."

Azarenka, unseeded this year after her ranking slipped into the 40s following an injury-interrupted 2014, won the Australian title in 2012 and '13 and reached the quarterfinals last year. She opened the tournament with a win over 2013 semifinalist Sloane Stephens and then defeated No. 8-seeded Caroline Wozniacki '€” the U.S. Open finalist last year.

Rain showers forced the roofs on all three stadium courts to be closed early, but they opened for later matches when the weather cleared. (**)

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