Pramac Ducati's Johann Zarco claimed the top spot as free practice got underway at the final round of the MotoGP World Championship at Valencia in Spain, with title contenders Jorge Martin and Francesco Bagnaia occupying third and 13th respectively.
Bagnaia holds a 21-point advantage over Martin with 37 points available across the weekend in the Sprint race and final Grand Prix of 2023.
Frenchman Zarco, competing in his final round for the Pramac team before making the move to LCR Honda for 2024, went quickest in the final seconds to lead FP1 by 0.184s from the resurgent Fabio Di Giannantonio at the Ricardo Tormo circuit.
A MotoGP winner for the first time last weekend in Qatar, Di Giannantonio is poised to secure a premier class lifeline with the VR46 Ducati team after losing his place at Gresini to outgoing Honda star Marc Marquez.
With Luca Marini – 17th in FP1 – expected to fill the vacant seat at Repsol Honda left by Marquez, Di Giannantonio is set to continue on Ducati machinery next season in a sensational turnaround for the 25-year-old Italian.
Championship challenger Martin made a strong start to the title-deciding weekend in third on the Pramac Ducati, 0.259s off Zarco's best lap, while Bagnaia made some progress on the Lenovo Ducati to climb to 13th after languishing in 21st earlier in the session.
Spain's Martin set the early pace with a 1:30.877s to lead from Marco Bezzecchi (VR46 Ducati) and Alex Marquez (Gresini Ducati) with lap times already under the race lap record in the early part of FP1.
Repsol Honda's Joan Mir had a big crash after 10 minutes at Turn 9 as he bounced into the gravel, while Alex Marquez went down at Turn 1 moments later with the cooler temperatures catching some riders out. Mir was sent to hospital for checks.
Marc Marquez hit the top with a 1:30.564s with 20 minutes remaining as the Spaniard enters his final weekend as a Honda rider ahead of his move to Gresini Ducati. The 30-year-old will ride the Desmosedici for the first time in Tuesday's post-season test at Valencia.
Marquez was then bumped down to second by Aprilia's Maverick Viñales, who went faster by 0.030s with a 1:30.534s.
Di Giannantonio fired in a 1:30.378s to return to the top on the Gresini Ducati
Martin improved his time in the final moments to go second behind Di Giannantonio by 0.072s with Bezzecchi and Zarco next as Ducati filled the top four places.
However, Zarco produced a fast late lap in 1:30.191s to hold sway by 0.187s from Di Giannantonio, with Martin 0.259s off the top in third.
Bezzecchi, Viñales and Yamaha's Fabio Quartararo were the top six while Marc Marquez finished seventh.
Augusto Fernandez (GasGas Tech 3 KTM), Enea Bastianini (Ducati Team) and Alex Marquez completed the top 10.
Japan's Takaaki Nakagami had a crash at Turn 8 on the LCR Honda with a few minutes remaining.