Four wins from the opening six races will have left many thinking that Casey Stoner was on an inexorable charge to the 2011 MotoGP World Championship. Jorge Lorenzo had just crashed out of the British Grand Prix and another accident at Assen left the Australian with a 28 point lead.

But the last two events have seen the pendulum swing back towards Yamaha's number one. The blow of being taken out in Holland was cushioned by his teammate Ben Spies who denied Stoner maximum points, while Lorenzo produced one the rides of his life to overhaul Stoner in Italy.

And last weekend at the Sachsenring, with Stoner following teammate Dani Pedrosa home to a Repsol Honda 1-2, the reigning champion dealt another psychological blow in the race for the title by mugging Stoner at the very last corner to snatch second place. So can Lorenzo force home his advantage in the United States?

This weekend would be as good a time as any with Yamaha celebrating the 50th anniversary of their first Grand Prix. Both YZR-M1s will be running in the commemorative red and white livery that proved so popular at Assen, but that is no guarantee of success stateside.

Based in Monterey, California, Laguna Seca is characterised by its unrivalled atmosphere and one corner in particular, a plunging left-right chicane that fills the riders with tension and apprehension every time it approaches, the Corkscrew.

The track features other fast sweepers that have caught out many a rider in the past but it's the Corkscrew that commands the utmost respect. The relief at making it through unscathed doesn't last long though as 1:20s later it is back upon them again. At 3.610km, it is the shortest circuit on the calendar.

The Corkscrew presents one of the great challenges on the Grand Prix calendar

Lorenzo was the victor 12 months ago, ironically after a head-to-head battle with Stoner, and this weekend may well be a repeat with Yamaha seemingly able to match the Hondas on race pace, finishing just over a second behind at the Sachsenring.

This will be music to the ears of Ben Spies as he gears up for his home Grand Prix, entering it for the first time with a realistic chance of victory. The Memphis rider will travel to the west coast of his native country to take on a circuit he has plenty of experience on, and plenty of pedigree too after guiding the Tech 3 Yamaha to sixth last year.

Stoner will also face a serious challenge from within his own ranks in the form of Dani Pedrosa. The diminutive Spaniard surprised even himself with his impressive victory last weekend, outracing both championship contenders. Like Lorenzo, Pedrosa is a former winner of this race and has made it clear that he isn't willing to support his teammate just yet.

Andrea Dovizioso sent out a very similar message at Mugello with his forceful pass on the championship leader but his own title hopes weren't helped by finishing a distant fourth in Germany. Consistency has kept the Italian in the hunt but he surely must start threatening the top step of the podium to avoid it becoming a two-horse race.

Valentino Rossi's Ducati career hit a new low at the Sachsenring when the nine-time world champion qualified on the back row of the grid. Fortunes have failed to improve on the GP11.1 and Rossi will have the GP11 available to him again this weekend.

Teammate Nicky Hayden will also have a choice of machinery with the 2012 chassis available to him for the first time. Whether he uses it is a different matter altogether but the Kentucky Kid is a Laguna specialist, with two of his three Grand Prix victories coming here, and will hope to get in amongst the frontrunners.

Nicky Hayden on his way to victory at Laguna Seca in 2006

Marco Simoncelli doesn't quite have the same affinity with the Monterey circuit having crashed three times over the course of his first visit last year. One year on and the Italian will surely have learned from his mistakes, even if his rivals may argue otherwise.

Cal Crutchlow will race at Laguna for the very first time and the Briton has made no secret of his excitement at taking on the challenge of the Corkscrew. He will hope to approach it with more grip than his YZR-M1 has offered lately. Colin Edwards managed to wrestle the other Tech 3 Yamaha to tenth in Germany and has shown impressive form since his return from injury. He is one man who won't be short of motivation this weekend.

The man heading to the U.S.A on a high must surely be Alvaro Bautista who fended off a pair of works Ducatis to finish an impressive seventh last time out, his best dry weather result of the season. The Rizla Suzuki squad were elated to be the best of the rest behind Yamaha and Honda and will be aiming to break into the top six here.

Four riders will have an immediate task on their hands to familiarise themselves with the track. Toni Elias and Karel Abraham were part of the inaugural Moto2 championship and therefore missed this event last year while Randy de Puniet and Hiroshi Aoyama were absent through injury.

Besides the factory machines, the highest placed Ducati in the championship is the Mapfre Aspar machine in the hands of Hector Barbera. The Spaniard will be hoping to maintain that record on the weekend of his 150th Grand Prix. Loris Capirossi will start his 322nd, assuming he has overcome his recent injury problems, although Sylvain Guintoli will again be on standby.

At the other end of the scale is Ben Bostrom who makes his Grand Prix debut at the age of 37. The AMA Superbike rider did the double at this circuit in World Superbikes ten years ago and gets his chance as a wildcard on a second LCR Honda.

The Red Bull U.S. Grand Prix weekend begins with FP1 on Friday evening at 6:10pm UK time and The Checkered Flag will have full coverage throughout the weekend.