Melbourne - Maria Sharapova continued to carve through the Australian Open draw with ruthless efficiency on Sunday but fifth seed Angelique Kerber crashed out as the heavy grind of the tournament's first week began to take its toll.
Sharapova was held up briefly when her serve deserted her in the first set but remained firmly fixed on her goal and beat Belgian Kirsten Flipkens 6-1, 6-0 to reach the quarter-finals, having conceded just five games.
"It's really about keeping your focus, no matter what the score is, no matter if you're up," said 2008 Australian Open champion Sharapova.
"I didn't start the match really great (but) then I started really getting under the ball and being aggressive and just had a little bit more energy which really helped me and I carried that throughout the match."
The second seed will next play compatriot Ekaterina Makarova in a rematch of last year's quarter-final after the world No 19 upset Kerber 7-5, 6-4 in the opening match of the day on the main showcourt.
German Kerber, who reached the semi-finals at Wimbledon last year but had never previously got as far as the fourth round in Melbourne, said a back injury had hampered her serving.
"I was thinking it would not be too bad, but in the first set it was getting worse and worse," she said. "I had treatment but it was not too good."
Makarova upset Serena Williams at the same stage last year before losing to Sharapova in the quarter-finals and said she was looking forward to the chance to turn the tables.
"Last year I was so surprised ... and had so many thoughts on my mind," she said. "This year, I'm a little bit used to it. I think I'll be ready to play a good game."
Where Sharapova has stormed through the women's draw, David Ferrer has moved unfussily through the men's toward a likely semi-final against defending champion Novak Djokovic, who plays Stanislas Wawrinka later on Sunday.
Seeded fourth in the absence of injured compatriot Rafael Nadal, Ferrer reached the quarter-finals for the third straight year after ending Kei Nishikori's challenge with a 6-2, 6-1, 6-4 victory.
"It's very important to get to the second week not feeling too tired and in good physical condition," said the Spaniard, who has dropped just one set all tournament.
"So the most important thing, I think, is not to play too many sets."
Eighth seed Janko Tipsarevic would undoubtedly agree.
After more than 11 hours on court at Melbourne Park this week, the Serb withdrew from his fourth round match against Nicolas Almagro with a foot problem while trailing 6-2, 5-1.
"I'm not completely happy because Janko was injured. That's not the way you want to win," said 10th seed Almagro, who will play Ferrer in an all-Spanish quarter-final.
"I'm hitting the ball with confidence and playing really aggressively. I need to be ready for a big fight."
Latest results from day seven of the Australian Open on Sunday:
Ekaterina Makarova (RUS x19) bt Angelique Kerber (GER x5) 7-5, 6-4
Maria Sharapova (RUS x2) bt Kirsten Flipkens (BEL) 6-1, 6-0
Li Na (CHN x6) bt Julia Goerges (GER x18) 7-6 (8/6), 6-1
David Ferrer (ESP x4) bt Kei Nishikori (JPN x16) 6-2, 6-1, 6-4
Nicolas Almagro (ESP x10) bt Janko Tipsarevic (SRB x8) 6-2, 5-1 ret