Ben McKinney gave England's selectors a nudge with a quickfire 214 not out as Durham piled up 456-2 after losing the toss at Gloucestershire.
A number of County Championship openers are on red alert ahead of England's first Test against New Zealand on June 4 and 6ft 7in McKinney put himself in the frame in stunning fashion in Bristol.
The 21-year-old had a triple-hundred opening stand with Alex Lees, who made 129, before becoming the club's youngest ever double centurion off 233 balls, and he is poised to go even bigger on Saturday.
Jamie Smith made his second ton of the season as he bids to hang on to his England place while the discarded Ollie Pope also made three figures in Surrey's 412-6 against Leicestershire.
While Dom Sibley was lbw to Josh Hull for six, Smith, who endured a wretched Ashes to leave his spot as England wicketkeeper-batter in doubt, followed up last week's 132 at Warwickshire with 166 on Friday.
Pope, meanwhile, got through the nervous 90s to make his first century since being dropped during the Ashes before he was out for 103 at the Kia Oval. Ben Foakes will resume on 62 not out.
Ben Duckett made a middling 25 in his first outing for Nottinghamshire this year before the county champions recovered from 103-6 to 279 all out against Glamorgan at Trent Bridge.
Duckett snubbed the Indian Premier League in an attempt to shore up his England position and made a bright start before getting a leading edge to mid-off before Jack Haynes led Nottinghamshire's recovery with exactly 100.
Josh Tongue bowled Zain-ul-Hassan and Fergus O'Neill took 6-2 in six overs as Glamorgan went to stumps on 81-4.
England managing director Rob Key was in Hove to watch Chris Woakes top-score to help lift Warwickshire from 117-6 to 267 all out against Sussex.
Out-of-favour seamer and Sussex captain Ollie Robinson and highly-rated all-rounder James Coles might have been hoping to impress Key but it was a former England bowler who outshone them both as Woakes made 64, sharing 115 with Kai Smith, who made 53.
Sonny Baker took three wickets in an over to leave Yorkshire on 48-4 after George Hill's five for 46 helped to skittle Hampshire for 251 at Headingley.
Baker pinned Sam Whiteman in front then got nightwatcher Ben Coad to chop on three balls later before a fearsome yorker from the England Test hopeful cleaned up James Wharton for a golden duck.
Shoaib Bashir claimed 4-76 - his best county figures - for new club Derbyshire as Lancashire were all out for 351 at Emirates Old Trafford. Derbyshire lost Caleb Jewell to close on nought for one.
Northamptonshire's top-three of Ricardo Vasconcelos, Luke Procter and Calvin Harrison all made centuries in their side's 409-1 against Kent at Canterbury.
Somerset stand-in captain Craig Overton took a couple of wickets as Essex were all out for 149 before making an unbeaten 50 off 36 balls - the only batter to reach a half-century at Chelmsford - as the visitors went to stumps on 179-5.
Middlesex closed on 91-6 after skittling Worcestershire for 191 at Lord's.
