John Isner fights past US counterpart Steve Johnson at Shanghai Masters

John Isner got the better of fellow American Steve Johnson in two tie-breaks to reach the third round of the Shanghai Masters this afternoon.

As is so often the case in matches involving big-serving Isner, there wasn't a single break of serve all game and both sets had to be decided in a breaker.

In the first, Johnson won four points in a row to go 5-2 in front but then lost the next four before Isner came through 8-6 after a run of six points out of seven.

It was a similar story in the second, with Johnson again taking a 5-2 lead but this time having two set points when 6-4 in front.

He couldn't take them, however, and Isner again battled through 9-7 to reach the last 16.

The 13th seed will next play the winner of the Feliciano Lopez-Rafael Nadal match that is currently taking place.