Mankad remained not out on 100 at stumps as Saurashtra recovered from a precarious 34 for three to end the day at 297 for 6, a lead of 97 runs with four first innings wickets in hand.
Table-toppers Karnataka had made 200 all out in their first innings.
Mankad's aggressive innings of 126 balls -- strike rare of nearly 80 -- was studded with 15 fours and a six. Kamlesh Makvana was the other not out batsman but in contrast to Mankad's brisk strike rate, his 30 came from 119 balls and was studded with just four boundaries.
Mankad and Makvana have added 101 runs so far for unbroken seventh-wicket partnership to put Saurashtra in the drivers' seat by reaching to 297 for 6 after starting the day at overnight 19 for 2.
Opener Snell Patel was the other main contributor for Saurashtra with a 155-ball 87 which was lashed with 13 fours. Patel was the last Saurashtra batsman out.
Karnataka removed Sheldon Jackson early in the day, reducing Saurashtra to 34 for 3. But overnight batsman Patel put on 72 runs with Jaydev Shah (39) to stabilise the innings.
Patel then put on 57 runs with Mankad, to bring Saurashtra to within five runs of taking the lead, before falling to Abrar Kazi (2/66) for 87.
For Karnataka, captain Vinay Kumar took 4 for 43.
Karnataka 1st Innings: 200
Saurashtra 1st Innings: 297 for 6 in 94 overs (Prerak Mankad 100 batting, Snell Patel 87; R Vinay Kumar 4/43).
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