After returning to Europe last month, the Formula 1 circus heads back beyond the continent's eastern boundary again this weekend, as it arrives in the eclectic city of Istanbul for the Turkish Grand Prix - what is promising to be the most open and unpredictable race of the season to-date.

Having triumphed in five of the opening six grands prix of the campaign, Jenson Button currently holds a commanding 16-point advantage over Brawn GP team-mate Rubens Barrichello in the standings, and is already hotly-installed as title favourite. The ex-Honda F1 outfit's star has not waned as some had predicted it would as the months wear on, and the Button-Brawn combo remains very much the one to beat in Turkey. It would be a brave man to bet against the Briton - a driver undeniably at the very apogee of his powers right now and making up for all the lost time of the past few years as he seemingly drifts inexorably towards the coveted crown - but there are all manner of drivers in his slipstream just clamouring to unseat him.

Chief amongst the baying pack, of course, is Barrichello, who has yet to truly get the better of Button in a race situation thus far in 2009, but is desperate to prove his detractors wrong and haul himself back into championship contention. Equally in with an excellent chance of spoiling the Frome-born ace's party is Sebastian Vettel who, after two relatively poor performances in Spain and Monaco, is under pressure to prove that he has the composure and nerve to match his undoubted raw pace and take the fight to the world championship leader. Red Bull Racing are expected to show well around the technically demanding and undulating, high-speed Istanbul Park Circuit - but its German driver has previously taken the chequered flag there no higher than 17th.

RBR team-mate Mark Webber will also be a threat, as will Ferrari duo Felipe Massa and Kimi Raikkonen, interestingly the only two drivers to have triumphed in Turkey since the country was welcomed onto the schedule back in 2005. The S?o Paulista, indeed, is something of a specialist around the track, remaining unbeaten both in qualifying and on race day for the last three years, and the recovering Scuderia was boosted by a much-improved performance around the narrow, tortuous streets of the glamorous Principality just under a fortnight ago. The challenge now is to prove that its Monte Carlo form was not merely a flash in the pan.

Beyond the front-runners, Toyota needs to rebound from a disastrous Monaco Grand Prix that delivered the big-budget Japanese manufacturer its worst-ever qualifying showing in some 129 outings in the top flight, with former race-winner Jarno Trulli and team-mate Timo Glock unenviably locking out the back row of the grid in a negative mirror image of their front-row lock-out just four weeks earlier in Bahrain. Much better is expected in Turkey.

Williams and Renault are likely to be found duelling over the scraps at the bottom of the leaders' table, with a point or two in the offering but - barring retirements ahead - little else looking achievable given their present level of competitiveness. That said, both Nico Rosberg for the former and Fernando Alonso for the latter can be counted upon to extract every last ounce of performance out of their cars, and will be worth keeping an eye out for on Sunday afternoon.

Traditional 'grandees' McLaren-Mercedes and BMW-Sauber have both been in the wars since the season began, but whilst the Woking-based concern has made considerable progress - albeit hidden in Monte Carlo by Lewis Hamilton's costly qualifying mistake - its Bavarian rival has conversely gone backwards, beating only the Toyotas last time out. By contrast, McLaren is anticipating a tough weekend ahead in Istanbul - a circuit that, like Barcelona, is widely-expected to highlight the inherent aerodynamic flaws of its MP4-24 - as BMW hopes that the introduction of a 'multi-level' diffuser will boost the F1.09 several rows up the starting grid and back into respectable contention.

Scuderia Toro Rosso, meanwhile, has struggled to exploit the full potential of its STR4 - on paper the same machine as Red Bull's podium-challenging RB5 - but is hopeful of a more consistent showing in the seventh round on the calendar, with rookie S?bastien Buemi keen to impress and namesake team-mate S?bastien Bourdais having received a timely injection of confidence from his point-scoring drive in Monaco. It will be fascinating to see, finally, what Force India can achieve, having climbed well away from the rear of the grid for the first time in Monte Carlo. In an era in which top eight finishes are increasingly hard to come by for the sport's smaller teams, a point or two for the Silverstone-based 'minnows' would undoubtedly be a popular result indeed...'s Tip for the Top: Sebastian Vettel's One to Watch: Felipe Massa