Brooks Koepka dominates professional golf at the moment. On Sunday, he won the PGA Championship, his fourth major win in the last 23 months. He has started eight major tournaments and has won 50 percent of them. Dominance might be an understatement.

Commentators who call tournaments on television and radio are all impressed by Koepka's monster drives, his physical fitness, and his mental game. "He doesn't seem rattled by anything," one expert remarked during CBS's television coverage. "He's unflappable," said another.

During a press conference this weekend, Koepka was asked if he pays attention to how his competitors are playing. He doesn't. "I'm just trying to do me," he responded.

Those six words capture the quality that helps Koepka maintain his famous stoicism. He said his resting heart rate is probably the same in a major as it is when he's relaxing on the couch. In other words, he doesn't allow negative thoughts or distractions to sap his mental energy. He focuses on the present and what he can control.

"I just stay in the moment," Koepka added. "I never think one hole ahead. I'm not thinking about tomorrow. I'm not thinking about the next shot. I'm just thinking about what I've got to do right then and there." It's "very simple," he says.

Focus on the only thing that matters now.

Koepka's right. The secret to maintaining a strong mental edge is simple--on paper. It's harder to put into practice, of course, but the principle is simple. People who execute a skill like hitting a golf shot, shooting a basket or, in a business setting, giving a presentation, often get in their own way.

If you obsess about a past mistake or worry about the future, you're taking your mind off the only thing you can control--how you perform in the here and now.  

Sports psychologists have found that failing to perform your best when the pressure is on is the result of a cognitive system jam--in other words, there are too many thoughts swirling in your head, most often negative ones. 

"In golf, when negative self-talk pops into players's pre-performance routines, simple putts go awry," Sian Beilock writes in her book, Choke. "The ability to control your thoughts and images during performance is crucial."

According to Beilock and other neuroscientists, the two thoughts that divert mental power are: anger/regret over a past mistake and worry about the future--two emotions that sap your brain's energy and do nothing to fuel your performance at the moment.

During the final nine holes of the PGA Championship, Koepka could have short-circuited had he let anger over past mistakes divert his attention from the task at hand. His commanding lead of seven shots at the beginning of the day shrank quickly when he had four bad holes in a row. No one expects to play Bethpage Black perfectly, since it's one of the hardest golf courses in America. But where most players would begin replaying their bad shots or worrying that they might not win the tournament, Koepka kept his attention focused on the only thing he could control: the shot he was about to take.

Koepka won by two shots and became the world's no. 1 golfer.

Keeping your mental edge means conserving mental energy. Regretting the past or worrying about the future saps your energy in the only moment that counts--now.