Daniel Ricciardo sounded a warning to Mercedes after the first practice sessions for Sunday's Chinese grand prix, declaring that they should watch out as Red Bull "are coming".
Mercedes, having won the opening three races of the season, still appear to be the dominant force in Shanghai as Lewis Hamilton topped the times in Friday's second practice session.
Team-mate and drivers' standings leader Nico Rosberg was third, but appeared to have been hampered on his fastest lap by yellow warning flags waved after Pastor Maldonado crashed his Lotus at the pit lane entrance. With a clear run, Rosberg may have bettered Hamilton's time.
The Silver Arrows were split by a resurgent Ferrari, driven by Fernando Alonso, after Spaniard had earlier set the fastest time in the first practice session.
Ricciardo's Red Bull was fourth, just half a second behind Hamilton with his teammate and world champion Sebastian Vettel in fifth, two-tenths of a second further back.
The encouraging session prompted Ricciardo to tell reporters afterwards that Mercedes should be aware that Red Bull "are coming".
But the affable Australian admitted that it would take a little more time.
"There won't be any deposing going on this weekend," Ricciardo said. "I would like to think that we have caught up, but I think they (Mercedes) have got a bit more in there that they didn't show today.
Ricciardo had said earlier in the week that he felt "pumped" for the Shanghai race, despite losing an appeal against his Australian grand prix disqualification and he talked enthusiastically about his chances for Saturday's qualifying session.
"We'd like second row in qualifying tomorrow. That's the aim at the moment," he said.
"We are making small improvements. The gap is a big one so it's not going to come down over one race, but I feel that we have crept up a little bit more," Ricciardo added.
"It's still a big one but we are coming."
By contrast Hamilton, despite setting the fastest time, was unhappy with the handling of his Mercedes throughout the day, complaining of understeer and later saying on team radio: "There's something wrong with this car. I'm coming in."
Rosberg said it wasn't only Hamilton having problems given the nature of the Shanghai circuit with long bends under braking.
"Lewis had understeer, I had understeer also, but everybody has that problem here because the circuit destroys your front tyres," said Rosberg.
"You just have to figure out a way round that. Turn one is a tyre killer because it never ends, it goes on and on and on."
Hamilton caused confusion when he was asked if the planned "clear-the-air talks with Rosberg following their acrimonious Bahrain battle had happened on Thursday night and replied: "No."
But Rosberg had a different take on the matter.
"Yeah, we sat down and went through everything," he said. "It's all behind us and it's flat out, full speed ahead."