Dustin Johnson says his golf game is in good form. Photo: Adam Hunger/AP

SHANGHAI - Dustin Johnson switched putters at the last moment and then nailed four birdies in his last four holes to seize a one-shot lead at the halfway stage of the $9.75 million WGC-HSBC Champions in Shanghai on Friday.

The world number one's spectacular nine-under-par 63 catapulted him over fellow American and overnight leader -- and good friend -- Brooks Koepka, who hung on for second on 132 overall.

Also in the hunt was Englishman Justin Rose, the Rio 2016 Olympic champion, and Thailand's Kiradech Aphibarnrat. They shared third on 135 at "Asia's major". Fifth was Patrick Reed of the United States, five shots off the lead.

Johnson, who triumphed in Shanghai in 2013 and has won all four prestigious World Golf Championships (WGC), followed up his opening 68 with a sparkling round that started well and only got better as the day wore on.

The 33-year-old had a couple of near birdie misses early in his second round, but found his range with back-to-back bridies on holes 12 and 13, then ramped up his challenge to finish with a flourish.

Johnson, tied 11th after the opening round, said: "Today obviously I played very well, but I actually got a putter like about 20 minutes before I teed off, it's a Spider, so it's one that I've used before.

"I wasn't really pleased with the one I played with yesterday so the guys here in China made me one," said the 2016 US Open champion.

"I got it right before I walked to the tee. I hit a few putts on the practice green with it and I was like, 'Oh, this will work pretty well'. 

"Went out and holed a lot of putts today, so kind of like it."

Koepka relishing Johnson showdown

Koepka, whose first major title came at the US Open in June, stayed in contention with a solid 68 at a sunny Sheshan International Golf Club.

The 27-year-old began his second round with real intent, reeling off two birdies on the trot to start the second day much as he finished the first.

Koepka said he was relishing doing battle with Johnson on Saturday, when they will be in the same group of players, and intends to make it a miserable weekend for his friend.

"I have no problem breaking Dustin's heart, I can promise you that, I'm sure he'd say the same thing if he was sitting up here," Koepka told a press conference with a smile.

"We both have a lot of fun off the golf course, but you know, when it comes to golf, I think we both want to kick each other's butts.

"There's no mercy out there, I don't think anybody wants to go home a loser."

Koepka added: "I hope it comes down to Sunday because there's not been a tournament where we've actually competed against each other." 

Roses's second round would have been even better but for a bogey at the 18th and he carded a 68 to go with his first-round 67.

"Listen, it's part of the 18 holes," Rose said of his last-gasp blip.  "Disappointing way to finish, felt like I was running out of steam a little bit coming in."

But defending champion Hideki Matsuyama, the world number four from Japan, again failed to fire.

He followed up his disappointing opening-round 74 with another two-over 74 as his challenge disintegrated.

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