Serena Williams is a major doubt

Defeated by an unseeded opponent Paul Oberjuerge wonders if the American has staying power for the big finals.

Serena Williams had previously won 17 matches in a row at the Australian Open.

Serena Williams remains capable of moments of dominance. Caroline Wozniacki can attest to that; the American casually crushed her 6-2, 6-4 in the US Open semi-finals last year.

It seems fair, however, to wonder if the 13-time grand slam champion still has the stamina and focus to last the seven rounds of a major. Williams, 30, lost the US Open final to Samantha Stosur in straight sets, and after three easy victories at the Australian Open she melted down against the unseeded Ekaterina Makarova in a 6-2, 6-4 loss yesterday.

Williams made 37 unforced errors, was broken five times and later said she would not have played, because of an ankle injury, had it not been a major.

The Australian Open represented only the eighth tournament for Williams since July of 2009. She missed nearly a year of tennis due to foot surgery and potentially deadly blood clots in her leg, and rolled her ankle at Brisbane two weeks ago, one day after revealing she "didn't love" tennis.

She credited Makarova with playing well but felt her poor performance was decisive. "I honestly think it was on my racket," Williams said. "I hit a lot of errors. I mean, she hit some great shots down the line. But … every ball that came, I just hit it as far out as I could."

Before arriving in Melbourne, Makarova had gone out in the first round of seven consecutive tournaments. She defeated Vera Zvonareva two days before ending Williams's Australian Open winning streak at 17 matches.

"I'm surprised because she's a great player and it's really tough to play against her," Makarova said. "But … I just feel so good and so focused. So I played my game and that's it. I won against Serena. That's amazing."

Petra Kvitova, the No 2 seed, advanced to the quarter-finals with a straight-sets victory over Ana Ivanovic, despite suffering the embarrassment of a missed ball on an overhead smash. Maria Sharapova also reached the final eight, as did the unseeded Italian Sara Errani.

Kei Nishikori of Japan produced the biggest surprise on the men's side yesterday, outlasting the No 6 seed Jo-Wilfried Tsonga 2-6, 6-2, 6-1, 3-6, 6-3. Nishikori, the No 24 seed, became the first Japanese male to get into the final eight of a grand slam since 1932.

Nishikori next plays Andy Murray, who spent only 45 minutes on court before Mikhail Kukushkin of Kazakhstan retired with an injury while trailing 6-1, 6-1, 1-0.

Novak Djokovic turned in another strong performance, defeating the Australian veteran Lleyton Hewitt 6-1, 6-3, 4-6, 6-3. Waiting for him in the final eight is David Ferrer, the No 5-seeded Spaniard who lost 6-2, 6-1 to Djokovic in the final of the Mubadala World Tennis Championship in Abu Dhabi 25 days ago.