Serena Williams loses to Ekaterina Makarova at Australian Open 2012

Williams had won her last 17 matches at Melbourne Park

Serena Williams became the most high-profile casualty at the Australian Open so far after suffering a shock last-16 defeat by unseeded Ekaterina Makarova.

The 13-time major champion - a five-time winner at Melbourne Park - made 37 unforced errors as 56th-ranked Makarova won 6-2 6-3 in 82 minutes.

"I don't know what to say. It's an amazing feeling," said Makarova.

In her first Grand Slam quarter-final, the Russian will play compatriot Maria Sharapova, who beat Sabine Lisicki.

Sharapova came from behind to beat her German opponent 6-3 2-6 3-6.

The 2008 champion, 24, won the opening three games before Lisicki reeled off the next six games to secure the first set.

But Sharapova fought back in the second set and then scrapped tooth and nail in the third game of the third set to hold serve before breaking 14th seed Lisicki in the sixth.

"I had lots of ups and downs so I was fortunate to finish on a high note," said the number four seed.

"I'm happy that although I didn't play my best tennis I hung in there."

The result means Sharapova retains a chance of finishing the Australian Open as world number one.

Current incumbent Caroline Wozniacki, Petra Kvitova and Victoria Azarenka, who are also all in the last eight, can also claim the number one spot.

For Williams though, her 17-match Australian Open winning streak was brought to an abrupt end.

The American, seeded 12th after an injury-hit 18 months, and absent from last year's tournament, had not lost in Melbourne since a defeat by Jelena Jankovic in 2008.

It was was her first loss to a player ranked outside the world's top 50 in three years and only the third time since 2006 she had lost before the quarter-finals of a Grand Slam.

A lack of match fitness seemed to catch up with Williams in the blistering heat and she was second best throughout as Makarova claimed a richly deserved victory.

She broke for a 3-2 lead in the opening set and then again for 5-2 as Williams's usually dominant serve faltered badly.

Williams improved at the start of the second set as she claimed a 2-0 advantage, but Makarova broke back immediately and then moved 3-2 ahead when her opponent double-faulted a remarkable four times.

The momentum remained with Makarova and despite squandering three match points, she came through on a fourth when Williams sent a tame backhand into the tramlines.

"She is an unbelievable player and it is very tough to play her," the Russian added.

"It was tough - it is a big stadium and an unbelievable feeling. I am just happy."