Marin Cilic had Alexander Zverev on the ropes in the first set, but lost to the German for the sixth time in a row.
Alexander Zverev continued his dominance of Marin Cilic with an error-strewn straight-sets victory at the ATP Finals on Monday.
Cilic led 5-2 in the first set of the Group Guga Kuerten opener at the O2 Arena, but squandered his advantage and lost two tie-breaks to go down 7-6 (7-5) 7-6 (7-1).
Zverev has now won six consecutive matches against the 2014 US Open champion since their first encounter in 2015, but the third seed was a long way off his best in London.
The world number five made 32 unforced errors but there were 46 from Cilic, with both players looking weary at the end of a long season as they attempt to reach the semi-finals for the first time.
— ATP World Tour (@ATPWorldTour) November 12, 2018
The German should have put away a backhand for three break points in a fifth game which Cilic won and gestured to his box after the fifth seed moved a game away from taking the opening set.
Cilic failed to serve out the set, though, making three unforced errors in a sloppy service game and inexplicably deciding not to challenge when his forehand clipped the line, but was called out.
Zverev took advantage of being let off the hook, winning a tie-break which included seven mini breaks with a backhand pass following a deft drop shot.
There were 46 unforced errors in that strange opening set and neither player was fluent early in the second, Cilic saving a break point to level at 1-1.
The Croatian had the initiative when Zverev dumped a backhand into the net to go a break down at 4-3, but it was back on serve immediately when he overcooked a forehand.
A weary looking Cilic saved a match point to take it to another breaker, but his forehand continued to misfire and he again opted against challenging when his second serve was wrongly called out as Zverev wrapped up his 55th win of the season.
Zverev  bt Cilic  7-6 (7-5) 7-6 (7-1)
Zverev – 20/32
Cilic – 17/46
Zverev – 8/4
Cilic – 8/4
BREAK POINTS WON
Zverev – 2/5
Cilic – 2/5
FIRST SERVE PERCENTAGE
Zverev – 63
Cilic – 51
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Zverev – 75/48
Cilic – 70/43
Zverev – 94
Cilic – 81