San Antonio - South Korea's Kim Si-woo fired a brilliant hole-in-one to surge into a four-shot lead at the Valero Texas Open in San Antonio on Friday.

The 23-year-old from Seoul, who had posted a six-under-par 66 to lead after Thursday's first round, delivered another dazzling outing at TPC San Antonio's AT&T Oaks Course to pull clear of a crowded field.

The highlight came on the 167-yard par-three 16th, when Kim uncorked a superb nine iron for only the second ace of his career.

That moment of magic helped turn a two-shot lead into a four-shot cushion, leaving Kim at 12 under with two to play. Two more pars completed a six-under-par 66 round.

Earlier Kim had bided his time, picking up two birdies on the front nine, with an 11ft birdie putt on the eighth putting him at two under for the day, eight under overall.

A 22ft birdie putt on the 12th took him to nine under, before a further birdie on the 14th left him on 10 under.

There was a minor wobble on the 15th when Kim twice found bunkers before recovering for par before his stunning ace on the 16th.

Kim will now look to hunt down his third victory on the PGA Tour following wins at the Wyndham Championship in 2016 and The Players Championship in 2017.

Attempting to thwart Kim's hopes of pocketing the $1.35 million winner's cheque are a chasing pack which includes Jordan Spieth and Rickie Fowler.

Spieth and Fowler are among six players tied for second at eight under, four shorts adrift of Kim.

Fowler had looked poised to grab a share of the lead after moving to nine under alongside Kim with five holes to play.

But back-to-back bogeys on the 15th and 16th sent him back into the pack before Kim stretched his lead with his hole-in-one.

Spieth meanwhile carded a second consecutive four-under-par 68 after a roller-coaster round which was blighted by three bogeys.

Spieth, teeing off on the back nine, carded two bogeys and a birdie in his opening three holes before adding three more birdies before the turn to move to two under for the day.

A bogey on his 10th hole of the day was followed by a birdie and five more pars. Then, on the par-five eighth, Spieth holed out from 113 yards for an eagle to move to eight under overall.

Joining Spieth and Fowler on eight under are South Korea's Lee Kyoung-hoon, Canada's Corey Conners and Americans Harold Varner III and Adam Schenk.

Leading second-round scores on Friday in the US PGA Tour Valero Texas Open at San Antonio, Texas (USA unless noted, par-72):

132 - Kim Si-woo (KOR) 66-66

136 - Jordan Spieth 68-68, Corey Conners (CAN) 69-67, Harold Varner III 70-66, Adam Schenk 70-66, Lee Kyoung-hoon (KOR) 69-67, Rickie Fowler 68-68

137 - Brian Stuard 67-70, Josh Teater 69-68, Matt Jones (AUS) 69-68, An Byeong-hun 69-68, Rory Sabbatini (SVK) 69-68, Scott Stallings 68-69, Jim Knous 70-67

138 - Ryan Moore 68-70, Jhonattan Vegas (VEN) 67-71, Graeme McDowell 69-69, Hank Lebioda 68-70, Scott Brown 71-67

139 - Abraham Ancer (MEX) 67-72, Peter Malnati 71-68, Beau Hossler 69-70, Scottie Scheffler 71-68, Roberto Diaz (MEX) 68-71, Charley Hoffman 71-68, Sam Ryder 73-66, Jimmy Walker 70-69