Luke Wright and Ben Brown were in record-breaking form as Sussex reached 441 for five against defending champions Durham in LV= County Championship Division One.
The pair came together with the hosts at 115 for five and remained there at the close having put on 326 runs.
The partnership is Sussex's highest for the sixth wicket, beating a record set in 1930, and the highest sixth-wicket stand for anyone against Durham.
Wright, on 183, is just four runs short of equalling his highest first-class score, while Brown ended the day unbeaten on 152, 40 more than his previous best.
The wicketkeeper, who is standing in for the injured Matt Prior, was dropped by his opposite number Phil Mustard before reaching his half-century.
Yorkshire were grateful for Tim Bresnan's lower-order runs to ensure England colleague Joe Root's good start was not wasted on day one against table-toppers Warwickshire.
Root made 69 and put on 91 with opening partner Adam Lyth, but the Tykes slipped to 181 for six before Bresnan (61) helped top-scorer Kane Williamson (75) add 113 for the seventh wicket as Yorkshire reached 310 for eight.
Neil Dexter led the Middlesex attack as he claimed six Lancashire wickets at Lord's.
Jos Buttler and Tom Smith each hit 59 as the visitors mounted a middle-order comeback before they were eventually bowled out for 266.
Dexter, more noted for his batting prowess, returned career-best figures of six for 63.
Michael Carberry hit an unbeaten half-century to put Hampshire in the ascendency on day one of their Division Two clash with Glamorgan at the Rose Bowl.
The England batsman racked up only his second score of 50 or more this season, ending the day on 62 not out in the hosts' 119 for one in reply to Glamorgan's 224.
Kyle Abbott, Matt Coles and Sean Ervine earlier claimed three wickets each, Ervine returning marginally the best figures of three for 36.
Captain Mark Wallace made an unbeaten 67 for Glamorgan but their total after losing the toss and being put in to bat looked well short.
Surrey's Matt Dunn returned career-best figures but Gloucestershire struck back with the ball to leave the clash at the Kia Oval finely poised after 16 wickets on day one.
Dunn, who celebrated his 22nd birthday on Monday, took five for 48 in Gloucestershire's total of 168 to eclipse his haul at Derby in 2011.
But James Fuller then took four wickets to reduce the hosts to 62 for six - with Zafar Ansari having also retired hurt - before Jason Roy and Chris Tremlett steadied things down to reach 118 for no further loss by the close.
The matches between Nottinghamshire and Northamptonshire and Derbyshire and Kent saw no play because of rain.