Fabio Quartararo warned of tyre problems with his Petronas Yamaha bike despite setting the fastest time in second practice for the Czech Grand Prix.
The French rider, who is chasing a hat-trick of race wins after successive victories at Jerez, heads the MotoGP championship by 10 points from Monster Energy Yamaha's Maverick Vinales.
And while Vinales suffered a crash in Friday's first practice session, returning to go fifth quickest in the afternoon, Quartararo was the man setting the standards again.
His fastest lap of one minute and 56.502 seconds was 0.007secs quicker than the time posted by team-mate Franco Morbidelli, who led the time sheets right until the closing moments of the second session.
Quartararo, 21, said: "I had a really bad feeling with the tyres. I'm feeling good on the bike but I don't understand, the tyre consistency is really bad."
He explained he felt his tyres had come up to scratch in Jerez, causing him to be puzzled by why it felt so different on the track at Brno.
"Honestly, every lap you lose two tenths [of a second] so we will need to understand what's happening because it's so weird," Quartararo continued.
"The first five laps we had a good feeling but then we are missing something. Tomorrow, mainly, our goal will be to work on the consistency.
"So it was not an easy day, but at the end it's good to know where the problem is and we will see where we can improve."
He told the official MotoGP website: "The front [tyre] is okay, it's about the rear that we are missing a lot.
"We know where we need to work and we'll try to find a little bit the problem."
Miguel Oliveira (KTM) was third quickest, ahead of Johann Zarco (Avintia), Vinales and Joan Mir (Suzuki Ecstar).
Valentino Rossi, who is one away from becoming the first rider to achieve 200 podium finishes in the top category, was 12th fastest.
Defending champion Marc Marquez is absent this weekend, having also missed the Andalusia race last time out with the broken arm he sustained in the season-opening Spanish Grand Prix.
There was another casualty on Friday as Pramac Racing rider Francesco Bagnaia broke the tibia in his right leg after crashing in morning practice, ruling him out of Sunday's race and the following weekend's Austrian Grand Prix.