Karlovic books Washington ATP semi spot

Ivo Karlovic will play Steve Johnson in the semi-finals of the ATP Citi Open in Washington.

Karlovic hit 26 aces and saved all four break points in his 7-6 (7-4) 7-6 (8-6) over Jack Sock to reach the last four of the hardcourt tournament for the first time since 2007.

American Sock had a chance to force a third set, holding a set point at 6-5 in the second tiebreaker.

But the 13th-seeded Croatian Karlovic erased that with an ace, then delivered a service winner to get to match point and won when Sock's forehand clipped the net tape and sailed wide.

Johnson advanced with 7-6 (7) 7-6 (15) quarter-final win over top-seeded John Isner.

Trailing in a tiebreaker, Johnson managed to put a 235km/h serve into play and then decided to try something even more audacious: lob his two-metre-tall opponent.

Much to Johnson's surprise, the looping backhand worked, winning the point.

Hanging in there against the big-serving Isner, Johnson weathered a total of 12 set points and withstood 29 aces to win their all-American clash.