Mercedes again topped the timesheets in practice with Niko Rosberg and Lewis Hamilton finishing first and second respectively.
The duo are again expected to challenge at the top in qualifying Saturday, but their race pace has been a completely different matter, however, with both Hamilton and Rosberg struggling to manage their tyres.
Red Bull did not have a great time of it in practice on Thursday, with world champion and driver's championship leader Sebastian Vettel managing only the ninth fastest time.
"It is hard for us to say where we are right now," Vettel told the Formula One official website. "We made a big modification on the car after the morning run -- that's why we ran only one hour in the afternoon and with the softer tyres the time simply wasn't there.
"So all in all, not ideal. The good news is that we have a fast car when everything comes together. Now it is up to us to make everything come together."
Vettel does expect his Red Bull car to improve in Saturday's morning practice followed by the qualifying session.
"To be honest, we haven't had a shot at how to go about qualifying on Saturday," he said. "But Saturday morning (practice) is hopefully enough time to get a clue in which direction we have to set up the car.
"It is indeed a bit unfortunate that we didn't have more running in the afternoon and the red flag also didn't help, but this is Red Bull and we've always found an answer to the toughest challenge.
"We will sit down and analyse the data and we have a full day tomorrow to have some mind games on how we will approach qualifying on Saturday and Sunday's race."
Even if Mercedes might be looking strong, Vettel was quick to shoot down suggestions of the team being the favorites in Monaco.
"I can understand that people make out Mercedes as the favourite here given their qualifying performance in the last couple of races and we all know how important the grid position is here," he added. "But Thursday is much too early to make any predictions.
"It will again be a question of the tyres -- how long they last -- because a pit stop will put you way back and overtaking is practically impossible. Of course they are shooting for pole position -- just as we do."
To watch Monaco GP Qualifying session Live Streaming, for U.S. viewers, click here.
