The 26-year-old Spaniard won five of the first nine races in the front half of the season on his Honda, including the last one in Germany a month ago, and has built up a 58-point lead in the title race.
"We finished the first half of the season in a very strong way and now we must keep our focus to continue this," said Marquez.
"Brno is a circuit where lots of riders are often strong so we can't take anything for granted."
Marquez has chalked up two wins at the Czech circuit, in 2013 and 2017, and last year finished third behind the Ducatis of winner Andrea Dovizioso and Jorge Lorenzo, who has since become his teammate at Honda.
Three-time champion Lorenzo remains out of action after fracturing two spinal vertebrae during a horrific crash in practice at the Dutch GP. He is due to return at the British GP on August 25.
Marquez looks a certainty to push on towards his sixth championship title unless the Ducatis, who have two wins this season, can sort out the stability problem which has caused them trouble in a number of races.
In Germany Dovizioso, still hunting for his first world title at the age of 33, did not even make it into the second round of qualifying.
"Despite the fact that our position on the finish line at Assen and Sachsenring was acceptable, our feeling on the bike hasn't been the best in those circuits," said Dovizioso who did well to come through and finish fifth at the Sachsenring.
"Without a doubt, we need to stay positive. We've worked hard on each aspect during these three weeks and we're ready to tackle the second half of the season with great motivation."
Dovizioso's teammate Danilo Petrucci, winner of his home Italian GP, lies third in the championship, six points behind his teammate and 64 off Marquez.
Over at Yamaha, Valentino Rossi has not won at Brno for a decade while Maverick Vinales, winner in Assen, has never made it on to the Czech podium.
- Bright start for Quartararo -
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Their satellite stable Yamaha-SRT has benefitted from a string of strong performances from the rookie Frenchman Fabio Quartararo.
Just 20, Quartararo has already notched up three pole positions and two podiums before crashing early in the German GP and injuring his right arm and left shoulder.
"My arm and shoulder feels good now, and I've been doing some motocross to confirm it," said Quartararo.
"I'm really looking forward to seeing how it'll be during a race weekend again. My expectation will be the same as in the past few races, inside the top eight."
Alex Marquez, younger brother of Marc, goes into the weekend with an eight point lead over the Swiss Thomas Luthi in the Moto 2 title chase.
Just two points separate the Italian Lorenzo Dalla Porta and the Spaniard Aron Canet in Moto 3.
The Moto GP is due to start at 14h00 local time (12h00 GMT) on Sunday with the weather forecasters warning of rain and storms during the race.