Lewis Hamilton has ramped up the pressure on Valtteri Bottas by challenging him to prove his championship credentials.
Hamilton heads into Sunday's Spanish Grand Prix trailing his team-mate by a single point.
The Mercedes pair have two victories apiece, but Bottas heads the standings by virtue of his fastest lap at the opening round in Australia
Bottas, a winner last time out in Azerbaijan, continued his strong start to the campaign by completing a practice double at Barcelona's Circuit de Catalunya on Friday.
Hamilton was just five-hundredths of a second slower, with the Ferrari duo of Charles Leclerc and Sebastian Vettel a third of a second further back, third and fourth respectively.
Although Bottas has been quick out of the blocks in 2019, Hamilton does not traditionally come alive until the second half of the season.
Naturally, there are doubts as to whether the Finn will be able to last the course of a 21-round campaign.
"Winning the championship is not just about winning a couple of races," said Hamilton.
"It is about the whole season. It will be interesting to see how he goes in the harder conditions. We've only had four races so watch this space."
Vettel will be desperate to ensure it does not become a one-team fight for the title. But the Ferrari driver has already been cast 35 points adrift, and despite boasting an engine upgrade for the first round of the European season here, he finished practice a third of a second off the pace.
As such, Bottas could yet be Hamilton's closest rival for a sixth crown.
Bottas, 29, was fortunate to keep his seat at Mercedes after he failed to win a race last year.
Hamilton's number two performance engineer, Riccardo Musconi, has since been moved to Bottas' side of the garage.
Musconi, an important figure in Hamilton's recent title triumphs, was promoted by Mercedes to the role of the Finn's race engineer, filling the void left by Tony Ross after his switch to Formula E.
"That has been a nice helping hand [for Bottas] and he has learned a lot from that," added Hamilton.
"The things I was doing with my settings - which my former engineer is now encouraging him to do - has definitely helped him move in the right direction. So, the little step he has taken is to be expected.
"I now have a new engineer. We work well together but it takes time to build a relationship. It is not as strong as something that has been built over six years."
The Mercedes hierarchy are obviously keen that Hamilton's relationship with Bottas does not deteriorate in the same fashion as the Briton's did with Nico Rosberg.
Indeed, it was at this race three years ago where both Mercedes cars ended up in the gravel at Turn 4 after they collided on the opening lap.
In a toxic campaign, Rosberg went on to beat Hamilton to the title at the final race in Abu Dhabi. Rosberg promptly retired.
Hamilton, 34, said: "I have grown a lot since then. I am much stronger in and out of the car and much better at knowing how to manage my relationships.
"I don't have any doubt in my ability. I am down for whatever challenge, and whoever is going to challenge me. Everything is great within the team. The respect is there, so don't expect what you have seen in the past."