Spain's Rafael Nadal advanced to the second round of the Australian Open when his opponent, Australia's Bernard Tomic, retired due to an injury.

Nadal, the top seed, was up 6-4 in the match when Tomic was forced to stop playing.

The Australian player sustained a groin injury in practice earlier this week and requested medical assistance at the start of the match.

The 21-year-old Tomic played through the first set and was broken in the 9th game, giving Nadal the opportunity to serve for the set.

Tomic then told the chair umpire that he was retiring.

"I was like, 'Oh, no.' It's tough playing Rafa with two legs, let alone one but there was nothing I could do," Tomic told reporters after the match.

The 27-year-old Nadal said he felt bad that Tomic was unable to play in front of his countrymen, adding that he thought it was smart to retire and not aggravate the injury.

"I felt really sorry for Bernard. I was in that situation a few years ago and I know how tough is to take that decision. But if you feel bad, there is no reason why you have to continue. You put in risk the next tournaments for nothing," Nadal said.

Nadal's opponent in the second round will be Australia's Thanasi Kokkinakis, who beat Igor Sijsling of the Netherlands.

In other men's action, Switzerland's Roger Federer, the No. 6 seed, beat Australia's James Duckworth 6-4, 6-4, 6-2, while Argentina's Juan Martin Del Potro, the No. 5 seed, defeated American Rhyne Williams 6-7 (1-7), 6-3, 6-4, 6-4.

Canada's Milos Raonic, the No. 11 seed, beat Spain's Daniel Gimeno-Traver 7-6 (7-2), 6-1, 4-6, 6-2 and Italy's Andreas Seppi knocked off Australia's Lleyton Hewitt 7-6 (7-4), 6-3, 5-7, 5-7, 7-5.

Spain's Feliciano Lopez beat India's Somdev Devvarman 6-4, 6-4, 7-6 (7-2), while countryman Fernando Verdasco defeated China's Ze Zhang 5-7, 6-3, 6-2, 6-3.

Roberto Bautista of Spain beat American Tim Smyczek 6-2, 6-1, 6-1.

On the women's side, Victoria Azarenka of Belarus beat Sweden's Johanna Larsson 7-6 (7-2), 6-2, while Russia's Maria Sharapova defeated American Bethanie Mattek-Sands 6-3, 6-4.

Serbia's Jelena Jankovic knocked off Japan's Misaki Doi 6-1, 6-2 and Denmark's Caroline Wozniacki beat Spain's Lourdes Dominguez 6-0, 6-2.

Spain's Carla Suarez Navarro, the No. 16 seed, beat American Vania King 6-3, 6-2, while countrywoman Garbiñe Muguruza defeated Estonia's Kaia Kanepi 6-2, 2-6, 6-2.

The Australian Open, which is played on hard courts, is the first of four Grand Slam events on the tennis calendar. EFE