Middlesex captain Adam Voges suffered a suspected concussion at the Rose Bowl after being hit on the head by a throw while in the field against Hampshire.
Voges was helped from the field by both physios after the incident and will take no further part in the match.
Hampshire made 315-7 in their Division One clash, after being invited to bat first.
Opener Michael Carberry hit 107 and Liam Dawson an unbeaten 87
Half-centuries from Arun Harinath, Steven Davies and Jason Roy put Surrey into a strong position on 371-7 against Durham clash at the Kia Oval.
Roy stroked a fluent 64 from 87 balls after tea following a stand of 135 in 34 overs for the third wicket between Harinath and Davies, who made 96 and 87 respectively.
Durham seamer Chris Rushworth took two wickets in successive overs just before tea to send back both Davies and Harinath, but Roy then teamed up with Zafar Ansari (41) to add a further 112 in 25 overs for the fifth wicket and ensure Surrey took first day honours.
Davies scored a highly attractive 87 from 100 balls, with 11 fours, before falling to a brilliant diving catch at gully by Keaton Jennings. Then, in Rushworth’s next over, Harinath’s four-hour innings of 96 ended when he tried to pull a ball that was not really there for the shot and lobbed up a simple catch to mid off.
Harinath, who faced 188 balls and hit 16 fours, also produced some sparkling strokes in the sunshine after Surrey had claimed first use of an excellent surface. His first fifty included 10 boundaries.
Roy struck eight fours, but his downfall came from an ugly attempted flip into the legside against Graham Onions which resulted only in a gentle catch to mid on off a leading edge.
At Taunton Lancashire elected to toss and made the most of batting first, reaching 295-4 by the close.
There were contributions from all of the top six, with former Somerset batsman Alviro Petersen top-scoring with 83 and captain Steven Croft unbeaten on 71 at the close.
In Division Two England captain Alastair Cook continued his profitable start to the domestic season, reaching 53 not out for Essex against their hosts Worcestershire.
Having already scored two hundreds and a half-century he was only stalled by bad light on a day that allowed for only 40 overs.
Essex will resume on 144-1, Tom Westley unbeaten on 70.
Leicestershire bowled Sussex out for 163 at Hove, Ben Raine taking 4-30 and Wayne White 3-25.
The Foxes responded with 140-2, Paul Horton ending the day unbeaten on 71.
Mitch Claydon picked up 4-59 as Kent dismissed Glamorgan for 260 at Canterbury.
The Welshmen lasted only 64.5 overs, with home openers Daniel Bell-Drummond (62) and Tom Latham (48) helping their side to 124 without loss in reply.
Derbyshire and Northamptonshire were evenly matched at the County Ground, with Shiv Thakor’s 60no shepherding the visitors to 275 for eight.