In a protracted battle between the two Petroleum Sports Promotion Board cueists, Kolkata-based Kothari, a former Asian billiards champion, prevailed 4-31, 30-48, 89-37, 4-77, 83-50, 50-108, 55-56, 100-15, 79-40.
Two crucial breaks of 59 and 64 in the last two frames helped Kothari wrap up the match after he trailed Chandra, of Bharat Petroleum, who won the 2013 national snooker championship, 3-4 by dropping the seventh frame narrowly by one point.
Later, Kothari struck two timely half century breaks worth 59 and 64 to wrap up the last two frames and with that a semi-final slot.
Mehta, who survived a scare against veteran Yasin Merchant last evening in the pre-quarters, had things well in control when whipping hosts Maharashtras Shabaz Khan 5-1, an excellent break of 94 in the third frame being the highlight of his play.
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The other semi-final would pit Railways' Kamal Chawla, who knocked out cue ace Pankaj Advani yesterday, against Brijesh Damani of Indian Oil.
While Chawla brushed aside the challenge of Divya Sharma from Haryana 5-1, Damani ousted Sundeep Gulati of Delhi 5-2 in the quarter-finals.