Max Verstappen took back the Formula One world championship lead from Lewis Hamilton at Sunday's Turkish Grand Prix but champions Mercedes have got faster and alarm bells are ringing at Red Bull.
The 24-year-old Dutch driver finished second to Mercedes' Valtteri Bottas at the Istanbul Park circuit to move six points clear of seven-times world champion Hamilton, who ended up fifth.
At the same time Red Bull fell further behind Mercedes in the constructors' standings, the gap now 36 points, and team boss Christian Horner expressed surprise at how much his rivals had picked up the pace.
"I think their straight-line speed has taken a significant step recently," he told reporters.
"Whereas we could match them with smaller wings previously now we can't get near.
"We saw that particularly at this circuit where Lewis in particular had a significant straight-line advantage with a bigger rear wing on the car. We've got to maximise our package as best we can.
"It's surprising that they appear to have made the step that they have with the power unit."
Verstappen told reporters Red Bull needed to find more pace.
"They were definitely quicker this weekend," he said. "We just didn't get it together and also in the wet they seemed to have a bit of the edge as well.
"I definitely do think they probably stepped it up a bit," he added.
Hamilton had been fastest in qualifying in Istanbul and would have started on pole position had his car's engine not been changed as a precaution, triggering a 10 place grid penalty.
"We haven't done anything to the car so I don't really know why that is the case," the Briton, who took his record-extending 100th career win at the previous race in Russia, said when asked about the improved performance.
"Maybe the two tracks we've just driven on suited us a little bit more. The car's been feeling good, it felt great this weekend.
"If the car continues to behave as it has this weekend, that's good for us."
The last three circuits - Italy's Monza, Russia's Sochi and Turkey - have all been good ones for Mercedes in recent years but Red Bull have hung in there and come away with strong results to limit the damage.
While Mercedes-engined cars won all three races, it was Australian Daniel Ricciardo at Monza in a McLaren one-two after Verstappen and Hamilton took each other out.
Verstappen finished second in the other two, crucially ahead of Hamilton in Istanbul with team mate Sergio Perez third and Bottas no threat in the championship.
"Arguably this has been Mercedes' strongest circuit that we've seen so far this season so to have two cars on the podium and retake the lead in the drivers' championship is a positive day for us," said Horner.
"We know the next race in Austin has been a Hamilton stronghold for quite a few years now but then Mexico, Brazil hopefully, the higher altitude races have tended to be quite strong for us in the past.
"And then really not sure what to expect from the last three."
Qatar and Saudi Arabia, the races before the finale in Abu Dhabi, are both new to Formula One.