Jeff Gordon may be forced out of Sunday evening's Coca-Cola 600 marathon after suffering from serious back spasms since Thursday night's qualifying session.

Gordon cut short his participation in the first Sprint Cup practice session on Saturday and then skipped the second entirely, admitting that he would need to evaluate his condition to see whether he would be able to take part in this weekend's race, the longest on the NASCAR Sprint Cup calendar.

"Unfortunately I had a back spasm the other night in that last qualifying run that I made," Gordon told reporters afterwards. "I've been trying to get it worked out by today, and I got in the car and it just wasn't.

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"The good news is that the car is awesome, so I made 11 laps and it was everything I could do to do that," he admitted. "I just had to tell the team that I think it would be best if I sit out the rest of the day so that I can be prepared for this long, tough race that's going to happen tomorrow."

If Gordon feels that he's not able to undertake the race - or needs to pull out after the start - the Hendrick Motorsports team already has a substitute driver ready to fill-in.

"At this point we're going to have Regan Smith standing by for tomorrow," Gordon revealed. "But I don't think they're going to do anything more today. Again, luckily we had a good 11 laps there and we've got our teammates and information that we're going to be able to gather from them. It's all coming together very fast right now.

"It's unfortunate," said the 42-year-old who has four Cup championships to his name. "I've had some spasms in the past, but this one is a little bit different.

"I just want to really be cautious and take care of it," he said. "It doesn't do me any good to be in the car right now; especially when the car is as good as it is. It's really about getting prepared for 600 miles tomorrow.

"I have no doubts that I can be in this car and be competitive tomorrow if I just take it easy over the next 24 hours," he added.

Gordon's most serious past back issues came after a vicious accident on the road course at Watkins Glen International in 2009 which at one point appeared to threaten his racing career. Gordon is under the care of Dr Jerry Petty, a Charlotte-area neurosurgeon and spinal specialist who works with many active NASCAR drivers. However those fears passed and Gordon has been looking rejuvenated in recent months, claiming his first Cup win of 2014 two weeks ago at Kansas Speedway.

"This is something that he's had before, that he has some experience with," said Gordon's crew chief Alan Gustafson. "Our plan right now is for Jeff to come here, get in the car, start the 600 and finish it and win it. That's what we're going to try to do."

But if he is called up to replace Gordon, then Regan Smith will be a safe pair of hands for Hendrick. The 30-year-old New Yorker already has 151 Cup starts under his belt, but he's not raced in the series since this time last year. In the meantime he's been competing for JR Motorsports in the Nationwide Series, a team co-owned by Hendrick driver Dale Earnhardt Jr. that has a long-standing technical affiliation to the Hendrick organisation.

"I've tested with those guys on quite a few different occasions over the off-season, and I told them whatever they need, I'm there for them," Smith said. "Obviously, it's a great race car, great team and the main thing is going to be Jeff making sure he's healthy for whatever he needs to do the rest of the year."

Smith will be at work as usual on Saturday afternoon in the History 300 Nationwide Series race. He's currently in third place in the championship standings.