Dominic Thiem is convinced he can still challenge for the ATP World Tour Finals title despite opening his account with a loss.

Thiem upbeat despite Dimitrov defeat

Thiem upbeat despite Dimitrov defeat

The world number four went down 3-6 7-5 5-7 to Grigor Dimitrov in his first group match on Monday.

However, the Austrian felt his performance was an improvement on his recent displays and he remains confident he can still reach the knockout stages at the O2.

"It was I think way better performance than the last weeks, last tournaments, but still not perfect obviously," he said, as reported by Tennis World USA.

"[It] was normal first two sets, then the third set was a little bit weird I think when I broke back for 5-All, then immediately got a shocking break for 5-6, then still had chances. I think it was pretty open until the end. I think there were some things just not good enough.

"I think he deserved to win. It would have been better to win the match, of course. But still, I mean, there are two more matches, two more chances. I think it's just over when I cannot reach the semi-final anymore."

Thiem, 24, faces Pablo Carreno-Busta in his second match on Wednesday after he replaced the injured Rafael Nadal, and completes the group stages against David Goffin on Thursday.