* Isner out in Sydney
* Ferrer wins in Auckland
* Radwanska and Li set up semi-final (Adds Kerber win in final paragraph)
SYDNEY, Jan 9 (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.
World number five David Ferrer remained on course for his fourth Auckland Open title, albeit after a tough 6-4 2-6 6-3 battle against Taiwan's Lu Yen-hsun in his first outing of the year.
Agnieszka Radwanska, the women's top seed in Sydney, set up a semi-final against China's Li Na after both won their quarter-finals in much cooler conditions than the 40 degrees Celsius heat which threatened to suffocate the players on Tuesday.
Sydney men's second seed Gilles Simon pulled out before his last 16 match because of a neck injury, while Kei Nishikori and world number nine Janko Tipsarevic became the second and third players to quit the eight-man Kooyong Classic exhibition event.
All are expected to play at Melbourne Park next week.
World number 13 Isner, however, was hampered by a bone bruise to his right leg in his 6-4 6-4 defeat to qualifier Harrison and was left thinking about pulling out of the year's first grand slam.
"It's better. Definitely better than I thought, but it's not 100 percent," the American told reporters. "I don't know. I've 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 Mayer and setting up a quarter-final against defending champion Jarkko Nieminen.
"He's one player I don't like playing," Tomic said of German Mayer. "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.
"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 setbacks but the Argentine world number seven 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.
World number four Radwanska joked that it was "freezing" compared to the previous day's heat after she stormed past Roberta Vinci of Italy 6-4 7-5 and into the semi-finals.
The Pole has now won seven straight matches to start the year after her title win in Auckland last week.
"I'm very happy ... I have been able to play my best tennis from the beginning of the year," she said.
Li, the 2011 Sydney winner and runner up last year, joined her after fighting back to beat American qualifier Madison Keys 4-6 7-6 6-2.
Slovakian Dominika Cibulkova earlier upset third seed Sara Errani 6-2 6-1 to reach the last four where she will meet second seed Angelique Kerber, who beat Svetlana Kuznetsova 6-3 7-5 in their quarter-final just before midnight local time. (Editing by Pritha Sarkar)