The Ferrari of Carlos Sainz was quickest in Saturday's final practice session for the Singapore Grand Prix as Max Verstappen and Red Bull again struggled for pace.
Verstappen's best was only fourth on the time sheets, 0.313 sec behind Sainz's hot lap of 1min 32.065sec.
George Russell's Mercedes and the McLaren of Lando Norris also crossed the line ahead of the runaway championship leader.
Verstappen's Red Bull teammate Sergio Perez, who won in Singapore a year ago, was only eighth fastest.
The Red Bulls have been unstoppable so far in 2023, winning all 14 grands prix, with Dutchman Verstappen taking the chequered flag 12 times, including the last 10 races in a row. His Mexican teammate Perez has won the other two.
But Singapore's Marina Bay street circuit is shaping up as the biggest challenge to Red Bull's unprecedented quest to sweep all 22 races this season.
On Friday, Ferrari dominated with a one-two at the top of the time sheets in both practice sessions as Verstappen and Perez struggled.
On Saturday, Sainz was quickest for the second consecutive session, though his Ferrari teammate Charles Leclerc could not match his pace from 24 hours earlier and was fifth, but only 0.003sec slower than Verstappen.
Lewis Hamilton's Mercedes was sixth with the second McLaren of Oscar Piastri also showing improved pace to be seventh, more than half a second better than Perez, who complained of handling problems for the second day running.
"The rear is still on edge," he told his race engineer on team radio.
"I'm still sliding on entries and then the front is feeling quite light under braking."
Verstappen, who had called Red Bull's Friday performance "unexpected", was again unhappy.
"I'm just struggling for rear grip. You see that last sector? I'm just drifting," he complained to the Red Bull garage.
Nico Hulkenberg, in a much-improved Haas, and the AlphaTauri of Yuki Tsunoda rounded out the top 10.
Qualifying takes place later on Saturday and will be all-important around Singapore's tight and twisty city centre street layout, which made its debut on the calendar in 2008.
Nine of the 13 grands prix staged in the city state previously have been won from the front row of the grid.
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