By Martyn Herman
LONDON (Reuters) - Marin Cilic's miserable record at the ATP Finals continued as he was beaten 7-6(5) 7-6(1) by Germany's Alexander Zverev on Monday -- his ninth defeat in 10 round-robin matches at the O2 Arena.
Almost a year to the day since Zverev marked his debut in the season-ending showpiece by toppling the big-serving Croat, the 21-year-old Zverev again prevailed in a scrappy contest to take his career record over Cilic to 6-1.
Cilic, ranked two places below world number five Zverev, will be scratching his head at how he allowed an opening set in which he dominated to slip through his fingers.
The 30-year-old had point for a 4-0 lead and another one for 5-1 but allowed Zverev to claw his way back from a 1-4 deficit to take the set on a tiebreak, sealing it with a backhand pass.
Breaks of serve were exchanged in the second set before Cilic was required to save a match point at 4-5.
Cilic survived more Zverev pressure at 5-6, winning a long baseline exchange at 30-30 to set up another tiebreak.
Just as in the day's first shootout it was Zverev who jumped into a lead against an edgy Cilic who double-faulted at 1-4.
Zverev nailed a trademark backhand winner down the line to earn five more match points and converted the first with a powerful first serve that Cilic could only fend into net.
It was Zverev's 55th match win of the year, more than any other player on the Tour.
Later in the Gustavo Kuerten group, world number one Novak Djokovic takes on American debutant John Isner.
(Reporting by Martyn Herman; editing by Pritha Sarkar)