(Reuters) - Daniel Berger overcame a slow start to the final round before pulling away for a three-stroke victory at the FedEx St. Jude Classic in Memphis on Sunday.

The 23-year-old, who is the son of former world top-10 tennis professional Jay Berger, served up a three-under-par 67 to clinch his first PGA Tour victory.

He finished at 13-under 267 at TPC Southwind, with Phil Mickelson (67), Brooks Koepka (66) and Steve Stricker (67) equal second on 270.

"It's the greatest feeling," Berger told Golf Channel.

"You work so hard to get here and to finally be able to put the trophy up ... I'm just thrilled right now."

Berger, who arrived at the event ranked 46th in the world, credited his sporting family background with helping him cope with the pressure of leading.

"I'm just a competitor. I got off to a terrible start today (with a bogey at the first hole) and I just grinded it out and was able to get some putts to fall."

Berger will head to Oakmont for the U.S. Open that starts on Thursday brimming with confidence, as will Mickelson after his joint runner-up finish.

Mickelson was not thrilled with his long game in the first three rounds, but said he was sharper on Sunday.

"I hit it a lot better today," he said. "It was important for me to get in contention and feel the heat, and to just get my game feeling sharp heading into next week."

"It's a course (Oakmont) that will severely punish poorly hit shots and reward well struck shots, so to have a little bit of confidence heading into next week is a big thing."

Mickelson, winner of five majors, has finished runner-up six times at the U.S. Open, the only major he has not won.

He is looking to become the sixth man to win all four modern majors after Gene Sarazen, Ben Hogan, Jack Nicklaus, Gary Player and Tiger Woods.

(Reporting by Andrew Both in Cary, North Carolina; Editing by Peter Rutherford)