Form guide: only Webber can stop Vettel

Thirteen laps into Sebastian Vettel's long-run on the option tyre around the Austin track during practice, it was beginning to look as if the rubber had given its best and he was into the grip drop-off phase.

Had that been the case, then it would be difficult to imagine how he might do a one-stop race - much the favoured strategy around here - and, that being so, then perhaps Lotus's Romain Grosjean might be able to give him and Red Bull team-mate Mark Webber a race.

Another Japan or Germany, perhaps, with the Lotus not quite as ultimately quick but close enough that it could use its superior tyre longevity to make the Red Bulls have to fight for it.

But then, after five slowish laps, Vettel unleashed a 1m41.9s - a full second faster than he'd been doing at the beginning of the stint. What were those slow laps about? Upon that question might hinge the difference between a two-stopping Vettel that is forced to race a one-stopping Grosjean or a Vettel that simply disappears into the distance on his one-stop strategy.

The hard prime tyre looks incredibly durable around here, to the extent that drivers were simply getting faster and faster into their stints as their fuel loads came down. If the option medium tyre could last, say, 18-20 laps, leaving the hard to do the remaining 36-38 laps, then one-stopping is perfectly feasible.

Webber looked like Vettel's only rival © XPB

Vettel's stint on his options was for 16 laps, the last of which was that 1m41.9s. It's looking ominously like Vettel's tyre durability is just fine and that Red Bull's raw one-lap pace (around 0.5s clear of Mercedes and, potentially, Lotus) will ensure he's perfectly placed.

If that turns out to be how it is, then we need to look to Webber to provide the challenge. He was just 0.1s adrift on the option-tyred low-fuel laps (though Vettel's time came five laps in) and fresh off the back of two poles there's no reason to suppose he can't make it three.

His long-run pace on the hard tyre was exceptional, but we don't have a direct comparison to Vettel, who concentrated on the medium. Where Webber invariably struggles against Vettel is keeping a feeling for the softer tyre if the degradation rate becomes at all marginal.

So yet another question hangs on how the RB9 is going to treat that option tyre. It's quite likely that Vettel's confidence level is not seriously affected by this question.

Grosjean's run on the medium averaged out very similar to Nico Rosberg's on the hard - suggesting the Lotus will hold its usual tyre deg advantage over the Mercedes and be a stronger race car, even if no faster in qualifying.

Lewis Hamilton's running was concentrated on the softer tyre and it did not pan out well, the car seriously off the pace by the end of its 16-lap run.

Lotus new boy Heikki Kovalainen performed respectably rather than sensationally. His long run on the harder tyre averaged 0.6s off Grosjean and although he was ahead of him on one lap pace, he was carrying 10kg less fuel and Romain had a problem with the front end, that was subsequently corrected for his long runs.

Ferrari, as ever, is struggling for traction but Fernando Alonso is nonetheless is getting a tune from the car. His long-run on the option tyre was around 0.4s adrift of Grosjean's, albeit running for fewer laps.

The car's single-lap pace suggests that he, team-mate Felipe Massa and the McLarens of Jenson Button and Sergio Perez will be vulnerable to the Saubers in getting through to Q3.

Vettel's quest to equal Alberto Ascari's nine-race consecutive grand prix victory by the end of the year looks on-course.