Phil Mickelson remained on track to become golf's oldest major winner as he held a two-shot lead at the halfway stage of his final round at the US PGA Championship.  

The 50-year-old had ended Saturday's action at Kiawah Island with a one-stroke advantage over Brooks Koepka, who is seeking to win the tournament for a third time in four years.  

Mickelson's slender advantage disappeared with a three-putt bogey at the opening hole, setting the tone for an uneven front nine that saw him record just three pars but still reach seven under.  

The undoubted highlight was a wonderful chip from a tricky greenside bunker by the fifth green that found the cup, delighting a crowd that sensed they could be witnessing history in the making. 

Playing partner Koepka also had his struggles, following up an opening birdie with a double-bogey seven at the second. He sat at five under through nine, the same score as Louis Oosthuizen. 

Abraham Ancer had shown how it was possible to go low on Sunday, carding the best round of the week with a blemish-free 65 that owed much to a fast start.  

The Mexican birdied four of his opening six holes before picking up a further shot prior to the turn, seeing him go out in 31 strokes. While he cooled off on the way back in, it was still an impressive display.  

While his charge came too late to mount a challenge for the tournament, Ancer feels his superb score is a further sign of how he is getting close to making a major breakthrough in his career.  

"I usually like golf courses that are going to be tough, it's not just going to be a birdie-fest and you have to grind it out and have to hit the ball well where you are supposed to," he told Sky Sports.  

"It's not that I don't care about other events, I try to think about every event the same and try to win every time, but I do feel my game is better for golf courses that are tougher." 

Phil Mickelson and Brooks Koepka both shot 70 Saturday to set up a mouth-watering final pairing at the US PGA Championship. 

At seven under par for the tournament, Mickelson holds a one-stroke lead over his countryman entering the final round at Kiawah Island's Ocean Course after saving par on 18 while Koepka bogeyed the last. 

The 50-year-old Mickelson is the fourth player aged 50 or older to lead a major after three rounds in the modern era, which began in 1934. 

The others were Tom Watson at the 2009 Open Championship, Greg Norman at the 2008 Open, and Julius Boros at the 1973 US Open -- none of whom ended up holding on for the win. 

Mickelson has been resilient this week in South Carolina, though, steadying himself Saturday after going bogey-double bogey on 12 and 13 to make par on the final five holes. 

While Mickelson's resurgence has excited the fans, Koepka remains a model of consistency at the PGA.

He has finished at least tied for fourth in 12 of the last 13 rounds at the major, and he could become the first player to win the same major three times in a four-year stretch since Watson won the Open in 1980, 1982 and 1983.

Mickelson will be shooting for his sixth major title and first since the 2013 Open, while Koepka seeks his fifth. 

Louis Oosthuizen, who shared the lead with Mickelson entering play Saturday, managed just three birdies on the day on the way to an even-par 72 that left him five under for the tournament. 

American Kevin Streelman (70) is at four under, while Oosthuizen's South African countrymen Branden Grace and Christiaan Bezuidenhout are at three under after even-par rounds of their own. 

Bryson DeChambeau (71) was unable to gain ground on the leaders and enters Sunday five back of Mickelson along with Gary Woodland (72) and Joaquin Niemann (71). 

Jordan Spieth matched Billy Horschel for the low round of the day with a 68, and he sits at even par for the tournament along with Rickie Fowler (69) and Keegan Bradley (72).

Reigning Masters champion Hideki Matsuyama fell from contention with a 76, putting him at one over with the likes of Shane Lowry (73), Padraig Harrington (73) and Ian Poulter (73).