Orissa all-rounder Biplab Samantray's fine spell kept defending champions East Zone in the hunt for a berth in the Duleep Trophy final. He claimed three wickets for 22 runs in 14.1 overs to have South precariously placed at 134 for 5 at stumps on the second day.
Earlier, East were dismissed for 267 in their first innings.
South Zone's hopes now rest on the last recognized batting pair of left-hander Amit Verma and wicketkeeper CM Gautham, who need to add another 134 runs to get the first innings lead.
Earlier, Saurabh Tiwary duly completed a century and scored a patient 145 off 267 balls. His partnership of 153 runs for the seventh wicket with Basant Mohanty (58) helped East post a decent enough total after being reduced to 66 for six on the first day.
Samantray, who is known for his ability to move the ball, got rid of the Karnataka trio of Robin Uthappa, Manish Pandey and Stuart Binny as South Zone were in a spot of bother after a solid stand of 69 between Uthappa (40) and Abhinav Mukund (19).
But within the space of 10 deliveries both Mukund and Uthappa were back in the pavilion. Mukund was bowled by Bengal left-arm spinner Iresh Saxena while Samantray got Uthappa, who looked ominous having hit seven fours, to edge one to wicketkeeper SP Gautam.
Ashok Dinda trapped Ramaswamy Prasanna leg before as South Zone were reduced to 71 for three.
Manish Pandey (31) and Amit Verma (22 batting) then added 44 runs as South looked like recovering from the sudden collapse before Samantray struck twin blows.
First he was able to castle Pandey and then trapped all-rounder Binny leg before off the very next delivery. But Gautham denied the bowler a hat-trick.
East Zone 267 (Saurabh Tiwary 145, Basant Mohanty 58, A Mithun 4-68, S Binny 3-46).
South Zone 134-5 (Robin Uthappa 40, Biplab Samantray 3-22).