AUCKLAND (AFP) - Ana Ivanovic continued her compelling form in tennis' Auckland Classic, as she swept into the semi-finals with a 6-2, 6-3 win over Japan's Kurumi Nara on Thursday.

She is joined in the final four by Venus Williams, Kirsten Flipkens and Jamie Hampton.

Ivanovic, the tournament second seed and world No.16, has not dropped a set all week.

She raced through her match against Nara in 76 minutes, dominating with her powerful first serve.

The in-form Serb will now play close friend and doubles partner Flipkens for a place in the final.

Third seed Flipkens had to fight back from 2-5 down in the second set to defeat Sachie Ishizu from Japan 6-4, 7-5.

The Belgian has also won every match this week in straight sets, and broke Ishizu twice late in the second set to ensure she retained her clean record.

The second semi-final on Friday will be an all-American affair after Jamie Hampton beat Lauren Davis in a marathon three-setter and Williams beat Spain's Garbine Muguruza 6-3, 6-3.

The fifth-seed Hampton and her practice partner Davis appeared to know each other's game too well, with 12 service breaks in their three sets which lasted two hours 14 minutes before Hampton won 6-2, 4-6, 6-4.