The Northern Ireland rider, who began the defence of his world crown in perfect style with a sublime double at Phillip Island in Australia in February, was under the lap record as he set his benchmark of 1m 33.573s on the Ninja ZX-10RR in the blistering heat in Buriram, with temperatures topping 34 degrees in the afternoon.
Yorkshireman Sykes was 0.210s adrift, with Aruba.it Ducati riders Marco Melandri and Chaz Davies third and fourth respectively on the combined times from the first two free practice sessions, as four tenths covering the leading quartet.
Rea, who leads the championship by 10 points from Davies after the opening round, said: “We focused on a race simulation at the start of the afternoon session and then at the end I just waited to set a time until the last laps. We did not want to put too many laps on my preferred tyre choices.
“We do not have a massive allocation of my favourite front tyres so I was testing the one-step softer front tyre, in readiness for Superpole. It wasn’t so bad but with more grip the bike worked quite differently on track,” added the 30-year-old.
“At the end of the race simulation, the bike was working really well. But when we added more grip was a bit different. So I will have to change some things in my riding style, to be honest, when I go on a time attack tomorrow in Superpole. All in all, we are pretty OK.”
Rea and Sykes are the only riders who have won races at the Chang International Circuit since the venue was added to the calendar in 2015, with Rea chalking up three wins and Sykes coming out on top in race two last year.
Eugene Laverty was 10th fastest on the Milwaukee Aprilia RSV4 (1m 34.518s), sealing his place in Superpole 2 on Saturday, which will take place from 06:55 to 07:10 GMT on Saturday, with race one scheduled for 09:00 GMT.