Gael Monfils came from a set down to defeat resurgent veteran Ivo Karlovic and win the Citi Open in Washington on Sunday.

Karlovic, 37, saw off Gilles Muller to claim the Hall of Fame Tennis Championships last week, in doing so becoming the oldest winner on the ATP World Tour since Marty Riessen in 1979.

And he had the opportunity to become one of the oldest men ever to win consecutive ATP titles with a victory on the outdoor hard courts of the William H.G. FitzGerald Tennis Center.

But it was his opponent who instead bucked an unwanted personal trend, recovering from losing the opening set to win a final for the first time in his career, overcoming the Croatian 5-7 7-6 (8-6) 6-4.

It was a sixth career triumph at ATP World Tour level for the entertaining but inconsistent 29-year-old, who has been a runner up on 19 painful occasions. 

After Monfils saved break point in the first game, the opening set stayed firmly on serve, neither player affording the other so much as a glimpse of a break until the Frenchman finally cracked to go 6-5 down.

Karlovic then saved three break points before taking the set at the second attempt.

It was a similar story in the second set, with no breaks or even break point chances until Monfils gave up his serve to fall 5-4 behind.

This time, though, the second seed broke back, Karlovic following up his booming serve by coming to the net, only to send a volley long, Monfils drawing level at 5-5.

The world number 17 then stepped it up on serve, holding to love for the first time in the set to take a 6-5 advantage.

Karlovic responded by holding to force a tie-break, but netted a forehand as Monfils claimed it 8-6, the veteran Croatian dropping a set for the first time in the tournament.

Karlovic was broken in the third game of the decider and it proved fatal to his chances, Monfils needing just one of two match points to record what had appeared an unlikely victory.