GALLE: Joe Root's 13-month wait for a test century, the longest of his career, ended on Friday with a masterful unbeaten 168 on day two of the first test against Sri Lanka as he revealed a change in mindset was behind his success.

Root dominated Sri Lanka's spin attack on a turning wicket in Galle to help his side to 320 for four, a commanding lead of 185 on the first innings that he will look to extend in search of a double-ton.

"In the last two years I have made too much of a big deal about it in my own mind, which has been to my detriment," Root told reporters in responding to a question about his failure to regularly turn half-centuries into three figures.

"So I tried to get it out of my mind and just get into one-on-one contests with each bowler and choose the right shots.

"My shot selection was good today, I managed to pick the right ball the majority of the time. It is about doing that more often."

Root has made a half-century on 67 occasions in tests, and converted 18 of those into hundreds.

He concedes at being frustrated that he has not managed it more often, but now has a fourth double-ton in his sights.

"I have been desperate to convert the 50s into big scores, but when I do get a 100 I generally make it count," Root said.

"It felt like a long time coming but the aim now is to try and make this one count and drive the first innings (total) as high as we can to bat once in this game."

Weather permitting, England look overwhelming favourites to win the first match of the two-test series, but Root warned things can change quickly in Galle.

"There has already been prodigious turn, so this test is only going one way. We need to be ruthless and drive home the advantage," he said.