Ivo Karlovic outserved his fellow Croatian Marin Cilic on Saturday to book a spot in his first ATP Tour final of the year against Gilles Muller at the grass-court event in 's-Hertogenbosch in the Netherlands.

The 38-year-old, whose best surface has always been grass due to his 2.11-metre frame, edged out former US Open champion Cilic 7-6 (7/4), 5-7, 7-6 (7/2) after two-and-three-quarter hours.

Luxembourg's Gilles Muller, 34, stands between Karlovic and a ninth career title, after he sprung a surprise by beating second seed Alexander Zverev 7-6 (7/5), 6-2 in the first semifinal.

Sunday's final will have the oldest combined age for two players since Ken Rosewall (42) and Ile Nastase (30) went head-to-head in Hong Kong in 1976.

The battle of two of the game's biggest servers saw Karlovic save a set point in the tenth game, before going on to take the opener in a tie-break.

But top seed Cilic, who has reached the Wimbledon quarterfinals for each of the last three years, hit back to force a decider thanks to the only break of the match at the end of the second set.

Karlovic, helped by 29 aces in the match, shored up his serve though and, despite missing a match point in the 12th game, cruised through another breaker to move into his 18th ATP final.

The left-handed Muller went into his match with Germany's rising star Zverev as a definite underdog, but edged in front in a first-set tie-break.

The world number 10 Zverev fell apart in the second to hand Muller a straightforward victory, as the top two seeds both bowed out of the tournament.