Daniel Ricciardo sounded a warning to Mercedes after the 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.
"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."
Hamilton fastest
Mercedes set the fastest practice times again at the Chinese Grand Prix on Friday, though rivals Ferrari and Red Bull may have closed the gap somewhat on Formula One's runaway leader.
Mercedes' lewis Hamilton, winner of the past two F1 races, was quickest around the Shanghai International Circuit in the afternoon session after a suspension problem limited him to only nine laps in the morning session.
Ferrari's Fernando Alonso, however, was just 0.141 seconds behind Hamilton under the watch of new team principal Marco Mattiacci, who arrived in Shanghai to take charge following Stefano Domenicali's resignation this week.
Alonso also set the quickest time during the morning practice.