Dustin Johnson says he needs to "step up my game" as his battle with Jon Rahm for the world number one spot continues at the Open Championship.

Two-time major champion Johnson briefly lost top spot to Rahm after the Spaniard won the U.S. Open last month.

But the American is back on top for the Open at Royal St George's having reclaimed first place without playing last week when Rahm finished seventh at the Scottish Open.

"The rankings, they're tough to figure out, but yeah, obviously if you play well, you get to number one," Johnson said after his practice round with Rahm at the Kent links course.

"I need to continue to play well if I want to stay there. 

"Obviously Jon has been playing really well lately. I need to step up my game a little bit."

Johnson has only made one top 10 in his past 10 events but believes his game is returning to how it was in a fantastic finish to 2020 and a strong start to this year.

"Yeah, obviously I had played really solid there really for about six, seven months," he said of the spell around his Masters win last November. 

"I feel like the game is starting to get back to where it was, just seeing a lot more consistency with the shots and the shapes. 

"I think that's probably just a little bit of the difference. Putting, I felt like I putted consistently well for that time period, so just been working on the putter a lot – that ultimately can be the difference."

Johnson came so close to glory when the Open was last played at this course in 2011, in contention before a double bogey at 14 as he ultimately finished in a tie for second and Darren Clarke took the title.

He added: "That was a long time ago, but obviously I have good memories here, and I did play well. 

"I do like this golf course. I feel like it's a tough golf course, a typical links course. For me, I feel like most of it's going to be driving. If I can drive it well, then I feel like I'm going to have a really good week.

"It was a little bit firmer in 2011. Obviously they've had a lot of rain and the rough is definitely a little bit thicker than it was back then." 

As well as that near miss, Johnson has had top-10 finishes in 2012 and 2016, with the prospect of winning his first Claret Jug an enticing one.

"It would definitely be right up at the top," Johnson said when asked where an Open win would rank for him.

"It's obviously a major. It's a tournament where I've been close quite a few times. I really like coming over here and playing. Yeah, it would be right up at the top with the rest of them.

"I think it's shaping up to be a great Open Championship."