Max Verstappen returned from his sick bed to set the fastest time in opening practice for this weekend's Saudi Arabian Grand Prix.
The double world champion's arrival in Jeddah was delayed by a day as he battled a stomach bug.
But he showed no ill-effects as he raced to the top of the time sheets in the first running of the weekend.
Verstappen finished half-a-second clear of Sergio Perez in the other Red Bull, with Fernando Alonso third for Aston Martin. Lewis Hamilton finished sixth for Mercedes.
Verstappen crushed his rivals in Formula One's opening round of the season in Bahrain a fortnight ago to kick off his championship defence with an emphatic win.
And the Dutchman's pace in the opening action of the weekend suggests he will remain the driver to beat.
Alonso took a podium on his Aston Martin debut in Bahrain and the 41-year-old was best of the rest here, albeit seven tenths behind Verstappen.
Alonso's team-mate Lance Stroll finished fourth as Aston Martin's impressive start to the new season continued.
Mercedes arrived in Saudi Arabia in crisis mode following a turbulent opening race.
Hamilton finished fifth in Bahrain – more than 50 seconds behind Verstappen – and then accused Mercedes of ignoring him on the development of this season's machine.
Speaking on Thursday, he said his Mercedes team needs "a kick" after conceding his chances of winning a record eighth world title are over for another year.
Hamilton ended the day's opening running 1.2 sec off the pace and a place behind team-mate George Russell, who finished fifth.
Carlos Sainz was seventh for Ferrari, with team-mate Charles Leclerc, who will serve a grid penalty on Sunday for changing engine parts, 11th. Lando Norris finished 20th and last for McLaren.
The second session gets under way at 8pm local time (5pm UK).
Meanwhile Saudi Arabia is preparing to stage two Formula One races, promoter Prince Khalid bin Sultan Al-Abdullah Al-Faisal has claimed.
The country is hosting its third F1 event this weekend in Jeddah, but an additional track is currently under construction near Riyadh.
F1 bosses struck a 10-year deal to race in Saudi Arabia, with the plan to switch venues from Jeddah to Qiddiya, a purpose-built venue 25 miles from the country's capital.
But speaking ahead of Sunday's race on the Red Sea, Prince Khalid, Chairman of the Saudi Motorsport Company said: "Saudi Arabia is a very big market and we have a very strong economy, so the idea of having two races in Saudi is doable.
"I would not be surprised if Saudi in the near future will host two races.
"Formula One is growing and there is a lot of demand here. Saudi is big and the regions are so far away. It is like the United States. There are three races in the US because the market is so big there and there is demand.
"The decision is not made and there are a lot of things that we need to consider. We don't know if it would be practical and feasible for Formula One Management and the teams, but we built this track in Jeddah to last, and in theory we could have two races."
The sport is back in Saudi Arabia, 12 months after a nearby missile strike overshadowed last season's race. The country has also been heavily criticised for its human rights record.
Asked earlier this week if he was unhappy to be racing in Jeddah, Lewis Hamilton cryptically replied: "It is open to interpretation.
"Hopefully everyone has a safer weekend. That's as much as we can do, right?"
Responding to Hamilton's remarks, Prince Khalid said: "I haven't spoken to him, and I have not met him. Everybody has an opinion, and we respect everyone's opinion.
"We think we have a great country, we are changing a lot and we are opening a lot so there is a lot of progress.
"The last thing we want is for someone to come to Saudi who is forced to race by a contract or doesn't feel comfortable.
"We are working very hard to satisfy everyone. It is a difficult task, but we will do our best and hopefully Lewis's opinion will change in a year or two.
"The majority of the drivers are happy to come here, and they are seeing the change and the development. Hopefully we can make Lewis, and other people who think the same, more comfortable in the upcoming years."
