Kolkata, Dec 18: Karnataka rode Karun Nair's brilliant unbeaten ton to be in the driver's seat by posting 294/8 against Vidarbha who struggled despite Rajneesh Gurbani's five-wicket haul on the second day of their Ranji Trophy semi-final here on Monday. The eight-time champions now lead by 109 runs replying to Vidarbha's 185 in the first innings at the Eden Gardens. Resuming on their overnight score of 36/3, Nair, who was unbeaten on six, braved poor form in recent times to bring up his 13th First Class hundred by working the ball through mid-wicket off Aditya Sarvate's delivery. He took 181 balls to complete his century. At stumps, the 26-year-old was batting on a chanceless 148 off 261 balls with skipper R. Vinay Kumar (20) for company. The duo associated for a 69-run unbeaten stand for the ninth wicket that took the lead past 100. Nair's innings was laced with 20 fours and a six -- the only of the innings -- over backward square leg with a late pull. This was his third century this season although his last five scores read 16, 10, 20 n.o., 62, and 15. CM Gautam (73 off 125; 8x4) supported Nair well besides Vinay Kumar at the end. At one point, Karnataka were 225/8 after Gurbani wreaked havoc in the post-lunch session. In-form Gurbani (5/90) grabbed his third consecutive five-wicket haul. The 24-year-old returned figures of 5/38 against Kerala in the quarter-final, days after notching up a career-best 6/113 versus Himachal Pradesh in their last league engagement. Karnataka looked to be in control of things from the first ball with Nair and Gautam (overnight 9) easing through the gears without hassle. The pair stitched together 139-run stand for the fourth wicket before Umesh Yadav (2/71) got his back as he tried to square cut but holed out to Akshay Wakhare at deep point. Gurbani then took three wickets in quick succession sending Stuart Binny (4), Shreyas Gopal (7) and Gowtham K (1) back in the hut. Aditya Sarvate (1/35) then took the wicket of Abhimanyu Mithun (10) who starred with the ball (5/45) on Day I as Karnataka were 247/8 at tea. Nair though was in a league of his own, holding fort at one end and milking all the bowlers around the park with equal efficiency. He celebrated his hundred by getting down to his knees and mock-shooting an arrow towards the sky. Vinay Kumar joined hands for an important knock hitting three fours en route his 20 that helped them extend the lead. Brief scores: Vidarbha First Innings 185; Karnataka 294/8 (Karun Nair 148 not out, CM Gautam 73; Rajnessh Gurbani 5/90, Umesh Yadav 2/71).