'All results possible' as Northants head into final day against Surrey
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Ricardo Vasconcelos is unbeaten on 51 from 98 balls while Saif Zaib is with him on 19 not out, with the pair's sixth-wicket stand so far worth 48.
Earlier, Tom Curran scored his maiden first-class century in his first red-ball appearance since April, 2019, hammering 115 from just 93 balls on the ground where his father Kevin excelled for so many years in Northants colours.
Veteran batter Hashim Amla scored his 57th first-class century as he posted 133, with the pair guiding Surrey to 421 all out, a lead of 82.
The final day is scheduled to start at 10.30am on Thursday, and Northants head coach Sadler said: "It was a great day of cricket but we are slightly behind the game so we need to dig deep and fight tomorrow.
"All results are still possible for sure.
"We've shown a lot of fight and character throughout the season and done that again today.
"We set the pace on day one but kind of let Surrey but in but have fought well.
"Curran took the game away in that instance and Hashim is world-class, one of the best to ever play this game and showed that.
"He ground us down and didn't get out of second gear; he played the same on 130 as he did when he first went in.
"He took the sting out of us to set the platform for Tom to play the way he did."
Earlier, Luke Procter hit 55 from 141 balls to close in on 1,000 runs for the season - he needs another 69 - and Sadler praised the former Lancashire, man as well as fellow left-hander Vasconcelos.
"Procter has been outstanding all year," said the Northants hea coach.
"He is always the voice in the outfield encouraging people on and I really hope he gets to 1,000 runs as he deserves it.
"Ricardo has had some ups and downs with the bat but we know he is a really fine player and we know that. He has shown a different side to his game tonight."