Spieth, who would join McIlroy in being three quarters of the way to a career grand slam with victory on Sunday, defied strong winds and torrential rain - which led to a short suspension in play in late afternoon - to card an erratic 69, which included three birdies, four bogeys and an eagle.

At six under par, Spieth led by two from Ryder Cup team-mate Matt Kuchar, with England's Ian Poulter, who was second at Birkdale when it last staged the Open in 2008, on three under alongside US Open champion Brooks Koepka.

McIlroy was five over par after six holes of his opening round and seemed destined for a fourth missed cut in five events until a stern talking to from caddie JP Fitzgerald prompted a back-nine rally.

The world number four came home in 32 with three birdies in the last four holes and maintained that momentum on Friday with birdies on the first, third and sixth taking him to the turn in 31.

That was an eight-shot improvement on Thursday and although he dropped shots on the 13th and 15th, a birdie on the 17th helped McIlroy climb into the top 10.

"That's right up there," said McIlroy, who equalled the lowest score in major history with an opening 63 at St Andrews in 2010 and opened with consecutive 66s on his way to victory at Hoylake in 2014.

" To be in after two days and be under par for this championship after the way I started, I'm ecstatic with that. I went out and believed in myself from the first tee shot, hit it within 60 yards of the green and went from there.

"I just wanted to continue with that good feeling from the last few holes (on Thursday) and went out with that positivity and trust in myself and just need to keep that for the next two days. I cant wait."

Spieth gave himself nine out of 10 following an opening 65, but admitted he needed some good luck to post a halfway total of 134, matching the record in an Open at Birkdale set by Craig Stadler in 1983.

"I give myself a B grade today," Spieth said. "I got pretty frustrated through the turn, hitting it into pot bunker after pot bunker and squeaking out pars somehow, but I thought we did a good job after the horn blew.

"It couldn't have been better timing and I could play the last eight holes almost as a new round. I didn't get everything I could out of yesterday's round and I got more than what I deserved today."

That included a mis-hit fairway wood on the 15th which got just a few feet off the ground, but ended up 15 feet from the hole to set up an eagle for the 2015 Masters and US Open champion.

"W e've been here before and we're not going for the first major, but it is the first Open and I'd be lying if I said I won't feel plenty of nerves," added Spieth, who missed out on a play-off at St Andrews in 2015 by a single shot.

"But w e've been moving in the right direction and I believe we can do this."

Poulter completed a 70 either side of the 14-minute delay caused by torrential downpours which left standing water on several of the greens, the 41-year-old carding 16 pars, one birdie and one bogey.

" It feels absolutely marvellous to be in contention," said Poulter, who came through final qualifying to avoid missing a sixth major in succession.

"Just walking from greens to tees was pretty special with huge galleries really pulling for me."

Scotland's Richie Ramsay matched Poulter's 70 to finish two under and expressed similar sentiments at finding himself in contention to emulate fellow Aberdonian Paul Lawrie in lifting the Claret Jug.

" It's pretty cool isn't it?" the 34-year-old said. "You grow up and practice having a putt to win the Open.

"I'm in a position to compete for it which is something I have needed to do to tick a box, to compete with the best players in the world. Whether I do that the next two days I don't know, but I have to give 100 per cent and try to be the best I can be and take it from there."