Top-seeded John Isner lost 6-4, 6-4 to fellow American Ryan Harrison in the second round of the Sydney International on Wednesday.

The exit of Isner, who was promoted to the No. 1 seeding after Jo-Wilfried Tsonga and Richard Gasquet withdrew, followed two more injury withdrawals: second-seeded and 2011 Sydney champion Giles Simon (neck) and sixth-seeded Radek Stepanek (back).

Isner, who withdrew from the Hopman Cup mixed-team competition in Perth last week with a right knee injury, lasted 71 minutes against the 64th-ranked Harrison.

The Australian Open starts Monday, leaving injured players little time to prepare for the season's first major.

In other matches, third-seeded Andreas Seppi of Italy beat John Millman of Australia 6-2, 3-6, 6-3; Australian Bernard Tomic, who beat No. 1-ranked Novak Djokovic last week at the Hopman Cup, advanced with a 7-6 (4), 6-2 win over fifth-seeded Florian Mayer of Germany; and Denis Istomin of Uzbekistan beat fourth-seeded Fernando Verdasco of Spain 6-3, 6-3.

In the women's draw, top-seeded Agnieszka Radwanska of Poland advanced to the semifinals with a 6-4, 7-5 win over Italy's Roberta Vinci and Dominika Cibulkova of Slovakia beat third-seeded Sara Errani of Italy 6-2, 6-1.

After winning the WTA event in Auckland last week, Radwanska has a seven-match winning streak.

"So far I feel good and I'm very happy that I can play good matches, already seven in a row," she said. "I think this year is the first time that I played two tournaments before (the Australian Open). I never did that before. So far it's good and I'm going to keep going."

Fourth-seeded Li Na of China played American qualifier Madison Keys and two-time Grand Slam champion Svetlana Kuznetsova took on second-seeded Angelique Kerber of Germany in night quarterfinals at Sydney Olympic Park.