Joe Root went past Alastair Cook as England's highest Test run scorer with an unconquered hundred against Pakistan, anchoring his team to 492-3 in the first Test in Multan on Wednesday.

The 33-year-old overhauled Cook's aggregate of 12,472 runs and became the fifth-highest scorer of all time after he drove seamer Aamer Jamal for a straight boundary to reach 71 just before lunch.

Root was still batting on 176 at the close on day three with his partner Harry Brook on 141 not out, taking England within 64 runs of Pakistan's first innings total of 556.

The pair added 243 runs for the unbroken fourth wicket, rebuffing Pakistan's bowling attack on a lifeless pitch that had punished England's bowlers on the first two days.

"I'm obviously proud but I still feel there's plenty more left to do and many more runs to get," said Root.

"I always think your best hundred is your next one," he said. "You've got to keep staying hungry and willing to find ways of contributing, ways of getting better."

"I'm sure I'll be looking back on it at some point when I'm finished and be very proud of it," Root added. "But we've still got an opportunity to go and win this game which is exciting."

Pakistan's head coach Jason Gillespie said his team needed early wickets on Thursday.

"We suspected the way England will look to get a lead and have a crack at us," he told reporters. "We have to come out in the morning and hit the ground running."

Root's memorable 481-minute knock included 12 boundaries. Brook amassed the same number and added a six despite suffering cramps.

Brook reached his sixth hundred with a single off part-timer Saud Shakeel from 118 balls, adding to his previous three in England's 3-0 series rout of Pakistan two years ago.

Root added 136 for the third wicket with Ben Duckett, who made a robust 84 and showed no effect from a thumb dislocation on Tuesday that forced him to bat at number four.

Duckett was the only man dismissed in the second session, trapped leg-before by Jamal after hitting 11 boundaries. (AFP)