Story highlights Djokovic wins ATP World Tour Finals opener

Beats Dominic Thiem in three sets

Djokovic has dropped to 2nd in rankings

Czech Republic wins Fed Cup

(CNN) Novak Djokovic's quest to regain his world number one ranking began with a battling three-set victory over Dominic Thiem at the ATP World Tour Finals in London Sunday.

Djokovic, bidding to win the prestigious title for the fifth straight time to topple Andy Murray from top spot, had to survive a scare against his Austrian opponent as he lost a tense first set tiebreaker, 12-10, having saved six set points.

In his frustration, Djokovic fired a ball into the crowd and earned a warning for unsportsmanlike conduct from umpire Carlos Bernandes.

.@DjokerNole comes back from a set down to open his 2016 #ATPFinals with a win, d. #Thiem for the 4th time. More: https://t.co/rrBYNfJZTA pic.twitter.com/JopifCXON7 — ATP World Tour (@ATPWorldTour) November 13, 2016

Pressed on the issue after the match, the irritated second seed responded caustically to the gathered media.

"It could have been (serious), yes. It could have snowed in O2 Arena as well, but it didn't. It is not an issue for me. It's not the first time I did it," he said.