Golf: British Open 2016: Phil Mickelson on top, Danny Lee, Steven Alker miss cut

Phil Mickelson of the United States smiles as he walks off the 18th green after completing his second round of the British Open Golf Championships. Photo / AP

Free-wheeling Phil Mickelson insists he feels no pressure as he threatens to become the oldest British Open champion in 149 years.

Mickelson retained the clubhouse lead at 11 under par but gave the field hope after following his recording-equalling 63 with a second-round 69 at Royal Troon on Friday.

The American is one stroke ahead of Swede Henrik Stenson, who jammed home eight birdies playing alongside Adam Scott, whose 73 left the Australian 10 shots behind.

Dane Soren Kjeldsen and Mickelson's countryman Keegan Bradley were also safely in the clubhouse one shot further back after each carding 68s.

Mickelson looked poised to run away with the Claret Jug after surging five shots clear with birdies on the fourth and seventh holes, then going to within a couple of centimetres of holing out for a spectacular ace on the famous par-3 eighth hole, known as the 'Postage Stamp'.

But after going 27 holes without a bogey, the 2013 champion recorded two on the back nine after errant drives in the increasingly wet conditions on the Scottish coast. The 46-year-old was none too fussed, though, as he remained on track to become the oldest Open winner since Old Tom Morris in 1867.

"I don't feel the pressure like probably a lot of players do to try to win the Claret Jug because I've already won it," Mickelson said. "That takes a lot of pressure off me. The desire to capture that Claret Jug puts a lot of pressure on.

"The fact I've done it relieves some of that. I would love to add to it, but having already done that was big."

Defending champion Zach Johnson is five shots off the pace after returning a one-under 70. Former Masters champion Charl Schwartzel was among the early movers in round two, a birdie-filled 66 sending the South African charging up the leaderboard to four under.

Americans Bill Haas (68-70) and Tony Finau (67-71) and Englishman Andrew Johnston (69-69) were also six adrift of Mickelson.

Kiwis Steven Alker and Danny Lee will mis the cut at six over and 13 over respectively.

-AAP