* Plucky Russian through to last eight for second straight year
* Kerber treated for back injury (Writes through with quotes, adds byline)
By Ian Ransom
MELBOURNE, Jan 20 (Reuters) - Ekaterina Makarova sprung a second successive fourth round upset at the Australian Open when the Russian completed a 7-5 6-4 victory over fifth seed Angelique Kerber on Sunday.
The 19th seed started brightly and survived a wobble to close out the first set before going on to defeat her injury-hampered German opponent, the highest seed so far to lose in the women's draw, in one hour and 32 minutes with an ace.
Makarova advanced to her second consecutive quarter-final at Melbourne Park after matching her run at last year's tournament when she sensationally defeated five-time champion Serena Williams, who was also hindered by injury.
"It's an unbelievable feeling," the 24-year-old Muscovite said in a courtside interview at a sunbathed Rod Laver Arena.
"I really like to play here, everything is so perfect. My team did a really nice job."
Wimbledon semi-finalist Kerber received treatment on and off the court for a back complaint, which she said had affected her for two or three days.
"I was thinking it will be maybe not too bad, but in the first set I was feeling that and it was actually worse and worse," added the 25-year-old, who had felt the most pain during her serve.
Makarova will now meet second seeded compatriot Maria Sharapova, who ended her run 12 months ago.
After eliminating Williams as a 56th-ranked outsider, lefthander Makarova crumbled against Sharapova in the following match last year, managing only five games.
"I think it will be really interesting and I really want to play against her," Makarova said.
"Last year I was so surprised ... and had so many thoughts on my mind. This year, I'm a little bit used to it. I think I'll be ready to play a good game."
(Editing by Patrick Johnston)