Soumya outplayed Odisha's Kiran Manisha Mohanty in a rook and bishop ending to emerge sole leader with 6.5 points, while two-time defending champion Mary Ann Gomes and Padmini Rout remained half a point adrift.
The shock loss gave a severe jolt to Swati's title hopes as she slipped to joint third position with top seed Tania Sachdev with 5.5 points each.
Monnisha stole the limelight as she spoilt Swati's brilliant run in a Sicilian Maroczy bind and kept the leader on the defensive.
Eyeing a hat trick of titles, local favourite Mary Ann kept her hopes alive as she downed Nisha Mohta.
More From This Section
Padmini too logged a full point with a win over 14-year-old Ivana Furtado of Goa in an advanced Caro Kan defence.
Top seed Tania was held to a draw by Bhakit Kulkarni in a game lasting 74 moves.
Results (round 9)
WFM G K Monnisha (TN) (3) bt WGM Swati Ghate (LIC) (5.5); WGM Bhakti Kulkarni (Goa) (3.5) drew IM Tania Sachdev (AI) (5.5) ; WGM Kiran Manisha Mohanty (LIC) (3.5) lost to WGM Soumya Swaminathan (PSPB) (6.5); Bodda Pratyusha (AP) (4.5) bt Michelle Catherina (TN) (2); WGM Padmini Rout (Odisha) (6) bt WIM Ivana Maria Furtado (Goa) (4.5); IM Nisha Mohota (PSPB) (3.5) lost to WGM Mary Ann Gomes (W.B.) (6).