Mercedes can contend again after improvements - Red Bull boss Christian Horner

BARCELONA, Spain -- While Mercedes were keen to downplay their improvements ahead of the Spanish Grand Prix, Red Bull boss Christian Horner believes the world champions' new upgrades could bring them back into the title fight.

Mercedes has struggled so far this year with a car which bounces aggressively when driving at high speed. The team has been a distant third behind Ferrari and Red Bull at the opening five races.

The team hoped new upgrades this week would help propel them back up the order and Friday saw a resurgence, with George Russell and Lewis Hamilton finishing behind Charles Leclerc in second practice.

More encouragingly, both drivers reported far less bouncing -- which is known as 'porpoising' -- from the car.

Mercedes boss Toto Wolff is remaining pragmatic ahead of qualifying, which is held on Saturday.

When asked if Friday's showing was a clear sign of progress, he said: "Progress, but not exuberance and ecstasy."

While Ferrari went quickest in both sessions, Red Bull had a much slower start to the weekend. Horner suggested he had seen enough from the Mercedes to think the team has taken a step forward this weekend.

"I have been saying all year that it's only a matter of time before they sort it out and it looks like they are on top of their issues, so they will become a factor in this championship," Horner said.

"They've had a strong day today by the looks of things. We have a few bits we need to tidy up but it is interesting, it is another dynamic."

Hamilton was in a very positive mood after climbing out of the car.

"It is positive, I am super happy," Hamilton said on Friday evening. "We are not the quickest but we are on our way."