The seventh-seeded Indian lost her quarterfinal to her Belarusian opponent 5-7, 2-6 in 100 minutes at the DLTA complex.
Ekaterina, who had ousted second seed Math-ilde Johansson of France in the second round, capitalised on an erring Su-nitha, who squandered several opportunities to win the first set.
Ekaterina now faces fifth seed Estonian Margit Ruutel, who defeated unseeded Hungarian Aniko Kapros 6-4 6-3, in the semifinal on Friday.
The other semifinal feature fourth seed Yanina Wickmayer of Belgium and eighth seed Naomi Cavady of Great Britain.
Yanina rallied to beat South African sixth seed Chanelle Scheepers 6-7 (5), 6-3, 6-3 and Naomi also came from behind to beat Carmen Klaschka of Germany 3-6, 6-1, 6-4.
Sunitha in final
Sunitha and French partner Aurelie Vedy reached the doubles final, defeating the Czech pair of Veronika Chvojkova and Andrea Hvojkova 7-5, 7-5.
The second seeded Indo-French pair will now take on top-seeded Chinese pair Chun-Mei Ji and Sheng-Nan Sun, who defeated Uberoi sisters Neha and Shikha 7-5, 6-3.
Rastogi in quarters
Karan Rastogi quelled the challenge of France’s Antoine Tassart to storm into the quarterfinals of the ITF men’s event at Abidjan, Ivory Coast, adds PTI from New Delhi.
The fifth-seeded Indian pipped his opponent 7-6 (7-3), 3-6, 6-1 in the second-round match, which was postponed due to rain.
The 21-year-old Rastogi will next face top seed Dutch Matwe Middelkoop, who beat Matthew Smith of Great Britain 6-1, 7-6 (7-5).