Serena Williams Loses to Ekaterina Makarova at Australian Open 2012

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 or Germany's Sabine Lisicki.

The result ended Williams's 17-match Australian Open winning streak.

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."