Sharapova, Serena clash as stakes rise at Open

Melbourne: The business end of the Australian Open begins in earnest on Tuesday when top seed Serena Williams faces old rival Maria Sharapova for a place in the semi-finals in a repeat of last year's title match.
In the men's draw defending champion Novak Djokovic, taken to five sets in the fourth round, will have to improve as he faces Japanese seventh seed Kei Nishikori while Roger Federer aims to continue his smooth progress against Tomas Berdych.
Sharapova will be desperate to avenge last year's final loss to Williams, although the omens are not good for the Russian.
She has lost her last 17 matches against the 34-year-old Williams and the American 21-times grand slam champion has been in dominant form so far in Melbourne after some pre-tournament injury concerns.
Williams remains wary though, saying such a one-sided statistic can work both ways.
"I think the person who's winning could definitely feel the pressure because there is a lot of expectations," she said. "The person who is losing X amount in a row could think 'I don't have anything to lose.'"
Djokovic's bid for a sixth Australian Open title was nearly sabotaged by Gilles Simon in round four and the in-form Nishikori will pose a serious threat as he attempts to prevent Djokovic reaching a 29th grand slam semi-final.
Nishikori beat Djokovic the last time they met in a grand slam, at the 2014 U.S. Open semi-final, and will need to reproduce that form on Tuesday.
"I think the biggest thing is he doesn't miss," he said. "He doesn't give you easy points, any free points."
Federer takes on a familiar foe in Berdych, with the pair facing each other for the 22nd time.
The Swiss is on a four-match winning streak against the powerful Czech, but has lost their last two grand slam meetings.
Berdych will look to pull off another major upset, as he did at the same stage last year when he ended a 17-match losing streak against Rafael Nadal.
Fourth seed Agnieszka Radwanska opens the action on Rod Laver Arena when she battles it out with 10th seeded Spaniard Carla Suarez Navarro.
The 26-year-old scrapped through to the quarter-finals after being pushed to her limits by Germany's Anna-Lena Friedsam in the previous round.