After Rilee Rossouw and Morne van Wyk recorded tons for the hosts on day one, the Titans pair made batting look easy as they pushed their side up to 416 for five by the close – only 22 runs behind of the home side’s first-innings 438.
Du Plessis was out shortly before the close for a fine 153 off 216 balls, his fifth first-class hundred, while Behardien finished with 140 from 230 and their partnership totaled a massive 272.
Heino Kuhn also made 75 and played a key role in helping the Titans recover following three early wickets, two from Dillon du Preez (two for 84) and one from Malusi Siboto (one for 74), that saw them reduced to 44 for three early on.
After resuming on three without loss overnight, the Knights quickly removed nightwatchman Paul Harris (four), caught in the slips by Boeta Dippenaar off Du Preez.
Tumelo Bodibe (11) and Kuhn put on 24 together before Du Preez struck again by bowling the opener, with Martin van Jaarsveld (five) cleaned up by Siboto.
However, Kuhn and Behardien steadied the ship by taking their side past 50 in the 15th over.
The South Africa limited-overs wicketkeeper raised his 50 in 62 balls, before helping take his side past 100 in the 45th over as the Titans went into lunch on 137 for three.
Just seven runs later, Kuhn was caught by Michael Erlank off Dean Elgar ending his 95-ball stay.
But Behardien and Du Plessis dug in, going past their 50s in 77 and 95 balls respectively, with their partnership passing 100 from 175 deliveries.
It was 248 for four at tea and the pair continued to pile on the misery for the Knights afterwards when they went to their centuries.
Du Plessis was first to reach three figures, getting there from 143 balls to maintain his rich run of form from the recently concluded 1-Day Cup where he recorded 544 runs in his 10 matches.
Behardien’s sixth first-class ton followed from 170 deliveries, before the partnership was eventually broken with what turned out to be the last delivery of the day.
Du Plessis had just reached his 150 from 199 balls when he picked out Siboto with Quinton Friend the lucky bowler, although the Titans sit in the driving seat.
Teams:
Chevrolet Knights – Dean Elgar, Reeza Hendricks, Rilee Rossouw, Boeta Dippenaar, Ryan Bailey, Obus Pienaar, Morne van Wyk (capt, wk), Michael Erlank, Dillon du Preez, Malusi Siboto, Quinton Friend.
Nashua Titans – Henry Davids, Tumelo Bodibe, Faf du Plessis, Farhaan Behardien, Martin van Jaarsveld (capt), Heino Kuhn (wk), Albie Morkel, David Wiese, Hardus Viljoen, Paul Harris, Rowan Richards.