The all-rounder helped the hosts fight back with a knock of 75 not out as they posted a first-innings total of 290 against Worcestershire at the Ageas Bowl.
In reply, the visitors had reached 40 for two at stumps and will resume on Saturday morning.
Despite losing five wickets in the afternoon session, Berg’s impressive scoring moved Hampshire into a decent position in the game.
The 37-year-old revealed he had to have patience during the first part of his innings.
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He feels the first hour on day two will be ‘vital’ to the game’s outcome – and backed his fellow bowlers to relish overcast conditions.
Berg said: ‘I was trying to work with Dawsy (Liam Dawson) for a little bit and assess the wicket.
‘I said to a few of the tailenders the wicket is pretty simple to bat on.
‘To go out and bat on a wicket like that, you know what the bowlers’ game plan is, which is to go for the stumps or the pads.
‘You have to try to play as straight as you can. The wicket was a bit slow, so it was about adapting and getting used to the pace.
‘I think we are just in front at the moment. It all depends on the first hour tomorrow – that’s vital in terms of the game.
‘If we can restrict them with the runs and nick one or two wickets early that will be great and we’ll be on a roll.
‘It was possibly a missed opportunity (not taking more wickets) but the wicket is going to be the same on day two and there’s something in there.
‘The conditions will suit our bowling attack – even Fiddy (Fidel Edwards) can pitch it up and nibble it – while the rest of us will stick to our task.’
James Vince also struck 75 from 74 balls for Hampshire. Meanwhile, debutants Hashim Amla and Sam Northeast fell for 36 and 17 respectively.
In reply, Edwards made the breakthrough in the second over when Brett D’Oliveira’s leading edge was easily snaffled by Vince.
And Kyle Abbott took his first wicket of the season when he clean bowled Tom Fell for 16.