SYDNEY (Reuters) - Top seed John Isner was knocked out of the Sydney International by compatriot Ryan Harrison on Wednesday but at least he got out on court on a day when injury withdrawals continued to ravage Australian Open warm-up tournaments.

John Isner of U.S. reacts during his men's singles match against compatriot Ryan Harrison at the Sydney International tennis tournament January 9, 2013. REUTERS/Daniel Munoz

Second seed Gilles Simon earlier pulled out before his last 16 match in Sydney with a neck injury, while Kei Nishikori and world number nine Janko Tipsarevic became the second and third players to quit the eight-man field at the Kooyong Classic.

World number 13 Isner was hampered by the bone bruise to his right leg that ended his participation in the Hopman Cup last week and the 6-4 6-4 defeat to Harrison left him thinking about pulling out of the Australian Open.

"It's better. Definitely better than I thought, but it's not 100 percent," he told reporters. "I don't know. I got a decision to make shortly.

"Certainly I could win a match maybe, a match or two. I don't see myself winning the whole tournament at this point, that's for sure."

Australian Bernard Tomic confirmed his return to form by riding his improved serve to a 7-6 6-2 win over fifth seed Florian Meyer.

"He's one player I don't like playing," Tomic said. "He's a great player, he beats the top four when he's playing well... I'm playing really good tennis."

Sydney fourth seed Fernando Verdasco was also a loser, 6-3 6-3 to Denis Istomin, while sixth seed Radek Stepanek withdrew injured after four points of his match.

Simon's withdrawal followed those of compatriots Jo-Wilfried Tsonga and Richard Gasquet, who pulled out because of a hamstring injury and for "personal reasons" respectively.

WRIST INJURY

"The week before the slams, players don't want to take any risks, which is normal but sad also for this one because Sydney is a very nice tournament and deserves to have a very nice draw," 2011 Sydney champion Simon said.

"We just try the maximum to be ready... I mean, for me today, there is no point in me going out and losing 6-2 6-2 playing 50 percent."

Former U.S. Open champion Juan Martin Del Potro knows all about injury and the Argentine got his 2013 season underway at Kooyong with a 6-4 3-6 7-6 round robin win over Paul-Henri Mathieu.

Japan's Nishikori, the world number 18, was forced to pull out with a knee injury, while Tipsarevic, a winner in Chennai last week, was a set down to Marco Baghdatis when he retired with a wrist injury.

There was better news on the weather front in Sydney with temperatures of around 25 degrees Celsius at the Olympic Tennis Centre as opposed to the 40-degree heat which threatened to suffocate the venue on Tuesday.

Top seed Agnieszka Radwanska joked that it was "freezing" compared to playing in her last 16 match after storming past Roberta Vinci of Italy 6-4 7-5 and into the semi-finals.

The Pole, ranked number four in the world and last year's losing finalist at Wimbledon, has now won seven straight matches to start the year after her title win in Auckland last week.

"I think I already had a lot of good matches already before Australian Open, so I'm very happy about that, that I could play my best tennis from the beginning of the year," she said.

Radwanska's semi-final opponent will be the winner of a later match between China's Li Na and American qualifier Madison Keys.

(Editing by John O'Brien)

• Please wait.

• O'Gara retires to take up coaching role
• Shields up to push claim for higher honours
• Dane Hoyer Larsen elected BWF president
• Crusaders slap down upstart Blues in soggy Christchurch
• Cycling: Leader Van Garderen wins California time trial
• Golf: Keegan Bradley maintains Byron Nelson lead
• Golf: Korda seizes lead at Mobile Bay LPGA
• Bradley stays in charge, Guan exits
• Hall of Famer Venturi dies aged 82
• Formula One: Increased venom as F1 tyre war erupts again