UPDATE 1-Golf-Fitzpatrick shoots 64 for second round lead in Memphis

* Firm fairways suit diminutive Englishman

* Cards a nine-under-par 131 halfway total

* Leads by two shots from four others (Updates at end of round)

July 26 (Reuters) - England's Matthew Fitzpatrick took advantage of firm conditions and stormed to the second round lead as overnight pacesetter Jon Rahm stalled at the WGC-St. Jude Invitational in Memphis on Friday.

Fitzpatrick birdied the first four holes en route to a six-under-par 64 at TPC Southwind, the day's best score and also his career-best in a PGA Tour event.

The former U.S. amateur champion posted a nine-under 131 halfway total for a two-stroke lead over Spaniard Rahm (71), Americans Billy Horschel (66) and Patrick Cantlay (68), and Australian Camerson Smith (68).

The diminutive Fitzpatrick delighted in the course's condition despite heavy rain early in the week.

"I have to admit, (I am a) little surprised at how the golf course is playing," the world number 29 told Golf Channel.

"I played Tuesday, gave myself absolutely no chance. It was soaking wet, I hit so many long clubs. Today it's a lot firmer a lot faster, a hell of a difference."

While Fitzpatrick surged, Rahm was unable to replicate Thursday's hot form on the greens when he sank five putts from outside 15 feet.

He had two early bogeys but battled to stay within contact.

Horschel bogeyed his final hole after a poor chip, while Smith also dropped a stroke at the last after finding water with his approach.

World number one Brooks Koepka and fellow four-times major champion Rory McIlroy carded 67. They trailed by four and five strokes respectively.

Sixty-three players are contesting the World Golf Championships event which has replaced the old PGA Tour event in Memphis.

The time slot, a week after the British Open, prompted five exempt players to decline participation -- Tiger Woods, Francesco Molinari, Rickie Fowler, Bernd Wiesberger and Open champion Shane Lowry. (Reporting by Andrew Both in Cary, North Carolina Editing by Christian Radnedge/Ian Ransom)