MELBOURNE, Australia -- Angelique Kerber said before the Australian Open that she wasn't putting too much pressure on herself as the top-seeded player at a Grand Slam tournament for the first time.
Clearly, she is still getting used to being No. 1.
The German player ended Serena Williams' reign atop the rankings after picking up her second Grand Slam trophy last fall at the U.S. Open following her breakout win at last year's Australian Open. Since then, however, her results have been pedestrian -- seven tournaments, no titles and a 5-7 record against top-50 players.
Her title defense in Melbourne also ended prematurely in a 6-2, 6-3 loss to American Coco Vandeweghe in the fourth round on Sunday. Hours earlier, Andy Murray was beaten in four sets by 50th-ranked Mischa Zverev, on the same court, meaning it's the first time in the Open era that both No. 1 seeds have been knocked out of the Australian Open before the quarterfinals.
"Of course, they are new experiences ... new challenges," Kerber said of being the defending champion and top seed at a major for the first time. "I can learn from all the other stuff which is new for me.
"I was trying everything, but I missed a lot and I make a lot of unforced errors. So this was not my game."
Kerber hasn't looked at all comfortable since the year began. She lost early in her two tuneup events for the Australian Open and struggled to find her rhythm in Melbourne once the tournament began, dropping sets in each of her first two matches.
Against Vandeweghe, the usually aggressive Kerber was simply overpowered, hitting just seven winners to 30 for her opponent. She was up a break in the second set and looked to be regaining control of the match, but Vandeweghe came charging back with pinpoint forehand winners that Kerber just watched go by.
Vandeweghe, a former Wimbledon quarterfinalist, often lacks consistency and can let her emotions and temper get the better of her, but she looked composed in the match and far looser with her groundstrokes than Kerber.
After serving it out, there was no celebration from the American -- she merely shrugged.
After ending 2016 on a four-match losing streak, Vandeweghe started the season with a new mindset -- she wanted to project more confidence on the court and play more freely. She set a goal of making the quarterfinals at the year's first Grand Slam -- and she's done just that.
"Going out there and playing an opponent, any opponent, I go out there expecting to win," she said. "It's just another person that's in front of me, whoever it may be, if it's No. 1 in the world, No. 130 in the world, it doesn't matter, it's still an opponent to get in my way of achieving my goals."
Kerber, meanwhile, will have to find her confidence again. She could lose her No. 1 ranking if Williams wins the tournament, which may help relieve some of the pressure she's felt as the favorite.
"It's just the beginning of the year," she said. "I can still improve my tennis, which is good."
Vandeweghe will next face No. 7 seed Garbine Muguruza.
Venus Williams, at 36 the oldest of the 128 women to line up for this Australian Open, is playing her 73rd major, the most among all active players.
Remarkably, she is through to the quarterfinals for the ninth time but only the second time in the past seven years. On Sunday, she dispatched German qualifier Mona Barthel 6-3, 7-5 with another solid effort.
"This is why you do all the hard work in the offseason," Williams, still breathing hard, said in her on-court interview. "She played well today. So many balls came back. I was really challenged."
History says the No. 13-seeded Venus is somehow turning back the clock. The last time she reached the quarters of a major without dropping a set was the 2010 US Open. The last time she reached the quarters here without the loss of a set was 2008.
Williams has lost to players ranked outside the top 100 only twice in majors, and one of them was four-time Grand Slam champion Kim Clijsters, who was unseeded at the 2009 US Open when she mounted a comeback after retirement.
The difference in this match? Both players created 10 break-point opportunities, but Venus converted four of them, as opposed to two for Barthel, who reached the fourth round of a major for the first time. Williams won 11 of the 17 rallies that featured nine shots or more -- surprising considering that Barthel is more than a decade younger.
As remarkable as her run has been, Williams has been fortunate with respect to the draw. Barthel is ranked No. 181 in the world and occupied the spot that would have gone to No. 4 Simona Halep, a former Grand Slam finalist, had she not crashed out in the first round.
No. 19 Kiki Bertens, who knocked Kerber out of the 2016 French Open, and Olympic gold medalist Monica Puig were also removed from Venus' path. In the third round, Venus defeated No. 87-ranked Duan Ying-Ying.
"I'm pushing forward to the future, and the past is something that I learn from and not something I dwell on," Williams said when asked about her longevity. "So that is how I see things. But honestly, it's just good scheduling. Throughout my whole career, I have never overplayed. And I think that saves you mentally and physically.
"I think I was born to play this game, I really do."
Williams added: "I've been blessed enough to do something I love. I grew so big and tall, I can cover the court. I was meant to do this."
Because they are on opposite sides of the draw, theoretically Venus could meet sister Serena in the final.
"That could hopefully happen," Venus said. "We both still have to work very hard to get there. Today I played a qualifier, and she hardly ever missed. So it doesn't matter who you come up against, they are coming and they want to win, too. They have nothing to lose."
In three of the past four majors, four American women made the round of 16.
Pavlyuchenkova then broke her fellow Russian's serve again at 3-all and once more in the final game, clinching the match in 68 minutes on her second match point when Kuznestova hit a forehand into the net.
"I'm kind of shocked a little,'' she said afterward. "But at the same time, so happy.''
Williams has a 3-2 edge in career meetings with Pavlyuchenkova, including wins in the last two matches.
The Associated Press contributed to this report.