Ducati rider Andrea Dovizioso topped MotoGP Czech Grand Prix FP1, but Marc Marquez, Maverick Vinales and Miguel Oliveira all finished less than a tenth of a second behind.
Marquez was the leader for nearly all the session, the Honda rider and championship leader having set a time of 1m56.948 early on to go close to half a second clear of the field.
The rest of the field could close the gap only in the last couple of minutes, with Yamaha's Vinales coming up just short in second as he demoted Suzuki test rider Sylvain Guintoli with a 1m56.948s.
Last year's Brno race winner Dovizioso set the timing screens alight on his final attempt and produced a 1m56.919s to finally depose Marquez from top spot by the slender margin of 0.029s.
Vinales was a close third, while Tech 3 KTM rider Oliveira grabbed an impressive fourth, just 0.099s slower than pacesetter Dovizioso.
Suzuki wildcard Guintoli was shuffled back to a still-impressive fifth in just his second appearance of the year, although he was nearly half a second off the pace.
Pramac Ducati's Jack Miller was second for most of the session, but didn't improve in the latter stages and dropped to sixth, followed by Oliveira's Tech 3 teammate Hafizh Syahrin.
The top 10 was completed by Franco Morbidelli (Petronas SRT Yamaha), Suzuki's Alex Rins and Valentino Rossi on the Yamaha.
Francesco Bagnaia was the only rider to crash, the Pramac rider going down at Turn 13 late on. He ended up 12th at the chequered flag ahead of Danilo Petrucci on the second of the works Ducati.
Fabio Quartararo on the sister SRT bike was a lowly 18th, while Jorge Lorenzo's replacement at Honda Stefan Bradl and Johann Zarco (KTM) rounded out the order in 22nd and 23rd.
|4||Miguel Oliveira||Tech3 KTM||KTM||1m57.018s||0.099s||17|
|6||Jack Miller||Pramac Ducati||Ducati||1m57.385s||0.466s||15|
|7||Hafizh Syahrin||Tech3 KTM||KTM||1m57.726s||0.807s||16|
|8||Franco Morbidelli||Petronas Yamaha||Yamaha||1m57.763s||0.844s||17|
|12||Francesco Bagnaia||Pramac Ducati||Ducati||1m57.950s||1.031s||14|
|14||Tito Rabat||Avintia Ducati||Ducati||1m58.029s||1.110s||17|
|15||Cal Crutchlow||LCR Honda||Honda||1m58.033s||1.114s||16|
|17||Takaaki Nakagami||LCR Honda||Honda||1m58.101s||1.182s||16|
|18||Fabio Quartararo||Petronas Yamaha||Yamaha||1m58.246s||1.327s||19|
|19||Karel Abraham||Avintia Ducati||Ducati||1m58.287s||1.368s||16|