Round-robin play began at the ATP World Tour Finals in London, and it was Novak Djokovic and Milos Raonic earning wins on Sunday.

Djokovic overcame a one-set deficit to defeat Domenic Thiem 6-7, 6-0, 6-2.  The world no. 2 took a hit from the Austrian, who used baseline power to dictate play and kept the veteran off balance, in the first set. After dropping the set in a hotly contested tiebreak, frustration bubbled as Djokovic angrily struck a ball into the crowd at the direction of his coaches Boris Becker and Marian Vajda.

 But in the second set, the Serb’s defensive prowess began to frustrate the 23-year old who misfired on a high number of unforced errors and it quickly became one-way traffic.

As the match progressed, Thiem could not solve Djokovic’s efficient movement on court and as he continued to attack, his unforced errors count rose. He would finish with 47 to just 28 winners. After digging the early hole, Djokovic righted the ship and took 12 out of the last 14 games to close out the match.

Despite the outcome, Djokovic was not in a pleasant mood afterward when asked about his reaction to his first set loss. “You guys are unbelievable,” he said. “You're always picking these kind of things. I'm close? I'm still not suspended, so if I'm not close, I'm not close. It could have been [serious], yes. It could have snowed in the O2 arena, as well, but it didn't. I'm the only player that shows his frustration on the court? That's what you are saying? It is not an issue for me. It's not the first time I did it.”

Meanwhile, Raonic defeated Gael Monfils 6-3, 6-4 in the second singles match. The Canadian answered any questions as to his injury status with an impressive straight-set performance. Raonic came into London questionable after suffering a torn quadricep muscle.

It was a match featuring two clearly different styles as Monfils had hoped to use his athleticism and unpredictability to his advantage against the power game of Raonic. But the world no. 4 was never threatened on his service games as the Frenchman failed to sniff a break point opportunity in the match.

Meanwhile, Raonic gained a service break early in the first set and midway through the second which was all he needed.  He assessed his play afterward, “I gave myself a lot of opportunities,” he said. “I played well. I took care of my serve very efficiently. All those things came together nicely. I competed well, had a good attitude and hit the ball pretty well as well.”

Raonic will face Djokovic next as play in Group B continues Tuesday. Meanwhile, Stan Wawrinka will face Kei Nishikori and Andy Murray meets Marin Cilic as Group A opens play on Monday.