Josh Cobb's maiden first-class century set Leicestershire up for a victory push against Middlesex at Lord's.
The 18-year-old's previous best was 44, but he hit three sixes and 13 fours to finish the day 120 not out.
Cobb struck up an unbroken 266-run partnership with HD Ackerman, who himself ended the day unbeaten on 190.
The duo's tally was a record for the sixth wicket against Middlesex, eclipsing that of India's Vinoo Mankad and Vijay Hazare 226 in 1946.
It also eclipsed the Leicestershire record for any wicket against Middlesex, the previous best of 236 set in 1976 at Lord's by David Gower and Brian Davison.
Middlesex's attack laboured as the visiting batsmen took the fight to their hosts on a docile wicket.
They started slowly however, as Jim Allenby and James Taylor were both trapped lbw leaving Leicestershire 209-5 having started the day on 184-3.
Ackerman, who became the fourth player this season to make 1,000 runs, hit the first boundary of the day in the 18th over and after Cobb survived a few hairy moments the visitors took control with the teenager completing is hundred in 255 balls.
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