CINCINNATI, Ohio: Serena Williams continued to roll at the rainy WTA Tour hardcourt tournament here yesterday, booking her semi-final berth with a 6-2, 6-2 victory over fellow American Amy Frazier.

Quarter-final action at the $175,000 event was disrupted by rain a handful of times.

Serena had to wait out the completion of top seed and defending champion Patty Schnyder's win over ninth-seeded Sania Mirza of India, which was delayed twice, before she got on court in the final match of the evening.

Once she did, however, she made quick work of Frazier.

Serena, a seven-time Grand Slam champion, is competing for the first time since the Australian Open because of a nagging left knee injury.

The absence has seen her fall to 139th in the world rankings. But her return has been impressive.

Serena has not dropped a set and has lost just 12 games in three matches.

She'll face a tough semi-final match-up with unseeded Russian Vera Zvonareva, another former top 10 player who is playing her best tennis of the season.

Zvonareva trounced fifth-seeded Jelena Jankovic of Serbia and Montenegro, 6-1, 6-1 to to move into the semi-finals.

Earlier this season, Zvonareva fell at the first hurdle in five straight tournaments. But she turned things around with a victory at Birmingham, England, last month.

However, Zvonareva's only win in four all-time matches against Serena came in a walkover in 2004.

In the top half of the draw, Schnyder was pushed in a rain-delayed 7-6 (9-7), 7-5 victory over the 19-year-old Mirza, who was playing in her first quarter-final this season.

Schnyder's semi-final foe will be fourth-seeded Katarina Srebotnik of Slovenia, who overwhelmed sixth-seeded Marion Bartoli of France 6-2, 6-1, to reach her first semi-final of the year.