Nico Rosberg edged Mercedes team-mate and title rival Lewis Hamilton to set the fastest time in first practice at the Russian Grand Prix in Sochi.
He was 0.065 seconds quicker than Hamilton on the new 3.6-mile circuit around the site of the Winter Olympics.
McLaren's Jenson Button was third, ahead of Ferrari's Fernando Alonso.
The session, and the weekend in general, is taking place in the shadow of the accident suffered by Jules Bianchi in Japan.
The Frenchman's Marussia team have decided to run only one car in honour of Bianchi, who is in hospital in Japan in a critical condition after suffering severe head injuries in a crash at Suzuka on Sunday.
The Sochi track, playing host to the first ever Russian GP, carries a strong resemblance to the now defunct Valencia street circuit and Hamilton and Rosberg were among the many drivers who ran wide exploring the limits at the plethora of right-handed corners.
Turn Two and Turn 13 - both at the end of long straights - were proving particularly challenging.
Despite the number of off-track moments, the surface proved to have more grip than initially suspected.
Button was competitive throughout the session, setting the fastest time of all in the first 30 minutes of the session, when drivers have an extra set of tyres which they then have to hand back.
At the time, Williams's Valtteri Bottas was second quickest, but the Finn's session came to an early end shortly afterwards when a tyre blanket malfunctioned and ruined the set of tyres he was planning to use for the remaining hour.
Bottas slipped to 10th place overall, while Button was beaten by both Mercedes drivers once the two title contenders got up to speed.
But the McLaren ended the session an encouraging 0.196secs off Rosberg's pace and 0.213secs clear of Alonso.
However, Red Bull team principal Christian Horner said he "did not expect Jenson to stay there for the rest of the weekend".
Championship leader Hamilton heads into the weekend 10 points clear of Rosberg following his victory in Japan, his eighth of the season compared to Rosberg's four.
The race at Suzuka was the first time all season one of the two men has passed the other on track in a race and made it stick, when Hamilton swept by to take the lead.
Rosberg has offered no excuses for his defeat, simply admitting that Hamilton coped better with the loose rear end the Mercedes had in Japan in the difficult conditions.
But he added: "I believe and know that if I do a good job here, I'll win on Sunday. I'm not in the past. I'm here."
McLaren's Kevin Magnussen was fifth, ahead of Force India's Sergio Perez and Toro Rosso's Daniil Kvyat, who is moving to Red Bull next season to replace four-time world champion Sebastian Vettel.
The German, who was 14th fastest in first practice in Russia - two places behind team-mate Daniel Ricciardo - is to move to Ferrari, although it has not been officially announced.
Ferrari's Kimi Raikkonen was eighth, ahead of Toro Rosso's Jean-Eric Vergne, Bottas and the second Williams of Felipe Massa.