Marin Cilic suffered more misery at the ATP Finals after a straight-sets defeat to Alexander Zverev.

The world number seven was edged out in two tie-breaks at London's 02 Arena in a 7-6 (5) 7-6 (1) loss.

Last year's Wimbledon runner-up has now won just one of his 10 matches at the end-of-season tournament, and that came against Kei Nishikori in 2016 when Cilic had already been eliminated.

It was also a sixth straight defeat inflicted upon Cilic by German rising star Zverev, and a lot of it was of the Croatian's making.

Cilic had break points for 4-0 and 5-1 in the opening set but could not take advantage, allowing Zverev to work his way back into the set with a break for 4-5.

Then at deuce Cilic consulted umpire Ali Nili before deciding against challenging a long call which, had he done so, would have earned him set point.

Cilic said: "I asked him if it was out and he showed me that it was out, clearly, so just I didn't – I was not going for a challenge. Even when I hit the shot I felt it was going to be close."

It was hardly the stuff of the cream of men's tennis. The players had hit just 19 winners between them, and made a combined 46 unforced errors, when Zverev took the set on the tie-break.

Cilic broke for 4-3 in the second but, ever the nervous front-runner, he promptly let Zverev straight back in and had to save a match point serving at 4-5.

Then in the tie-break Cilic was left to rue a wasted challenge earlier in the set which meant he did not have one left to overturn another bad call. "A few calls like that didn't go my way," added Cilic.

Zverev made a terrible start but grew into the match as Cilic began to wilt.

"I just wanted to fight back from every score," Zverev said.

"He was up a break in both sets but it was a very important match, you don't want to start the group stages with a defeat so it's a very important win."

The pace of the court has been a topic of discussion so far this week with both players seeming to struggle with the bounce at times.

Zverev added: "It's very fast. It's definitely one of the quickest courts we have on tour. It's tough to find your rhythm on it."