It was factory Ducati rider Scott Redding who set the pace in FP1 in the morning, progressively improving from his early 1m36.658s benchmark to end the session a tenth clear of Rea.
His final time of 1m34.591s stood fastest until the final five minutes of FP2, when Gerloff – having just secured a one-year Yamaha contract extension – lapped the TT circuit in 1m34.512s to provisionally take the top spot in the overall rankings.
But Gerloff’s position at the helm turned out to be short-lived, with Rea finding more time on his final run, ending the day fastest of all with a 1m34.391s.
The six-time world champion, however, didn’t have a smooth day at the Dutch venue and was nearly thrown off his Kawasaki at the exit of Turn 15 halfway through FP2.
Behind Rea, Gerloff and Redding ended up second and third respectively in the combined timesheets while Alvaro Bautista was an impressive fourth on the factory Honda thanks to a late effort of 1m34.741s.
Tom Sykes made it five different manufacturers in the top five, the BMW rider finishing just 0.012s adrift of Bautista in fifth.
Championship leader Toprak Razgatlioglu was not far behind in sixth on the factory Yamaha, while Lucas Mahias was the top independent rider in seventh for the Puccetti Kawasaki team.
Michael Ruben Rinaldi was unable to match the pace of his Ducati teammate Redding, finishing four tenths behind him in eighth but fractionally quicker than the second works Kawasaki of Alex Lowes.
The time set by GoEleven Ducati rider Chaz Davies in FP1 was good enough for 10th overall, as he outpaced the second BMW of Michael van der Mark and Honda’s Leon Haslam – the trio finishing just 0.005s adrift of each other.
Andrea Locatelli was the next best rider on the second of the factory Yamahas with a best time of 1m35.081s during the afternoon session.
Several riders and teams are notably absent from this weekend’s fifth round of the season at Assen.
BMW squad RC Squadra Corse – which fields a one-bike entry for Eugene Laverty – announced earlier this week that it will not make the trip to the Netherlands due to what it described as “internal restructuring”.
This was followed by the news that the returning Alstare team had split with the Gil Motor Sport outfit, forcing Christophe Ponsson out of the weekend. The management of the team will revert to Gil Motor Sport, with Alstare team manager Francis Batta advised against travelling this year by his doctors.
Elsewhere, Kawasaki satellite team Perdercini is down to one bike with Luke Mossey absent from the entry list. The British rider joined the squad for a one-off outing at his home ground at Donington Park after it parted ways with its original 2021 signing Samuele Cavalieri just three rounds into the season.