Kevin Anderson beat Dominic Thiem in straight sets
Thiem had highlighted his poor start to matches as his biggest problem of recent tournaments and he continued that form against Anderson.
The Austrian lost serve early on to trail 3-1 and from there he was always fighting an uphill battle.
A wild forehand swing saw the ball clip off the edge of Thiem’s racket, over the hanging scoreboard and towards the line judges at the far end of the court, which summed up his early performance.
The world No 8 gradually began to find his rhythm, but the first set was already beyond him, despite saving a further six break points.
Anderson, who became the tallest player ever to compete at the ATP Finals at 6ft8, at least until John Isner (6ft10) plays tomorrow, produced 15 winners to his opponent’s five.
The second set was much more even as the pair were strong on serve and headed straight for a tiebreak without a single break point on offer.
Thiem earned the first mini-break but Anderson quickly levelled following the longest rally of the match.
The duo continued to tussle as Thiem saved three match points before Anderson served up the 12-10 tiebreak win with an ace.
Thiem’s slow start to the match proved costly
Anderson now sits top of Group Lleyton Hewitt with Roger Federer and Kei Nishikori getting underway at 8pm this evening.
“It is just fantastic to be here. I have been watching this tournament since I was a little kid,” Anderson said.
“To get a win today I couldn’t be any happier now.
“I am so excited. I love being here. Hopefully I’ll be here for another week to come.”
Anderson was making his ATP Finals debut
Meanwhile, Thiem rued his poor start to the match.
“I didn’t have a good start,” he admitted. “I mean, at the beginning I was trying to return on a similar position than in New York.
“It was not working out at all. I mean, I had no chance basically in the first set to break him. Me, myself, I was in trouble almost every service game.
“So the first set was not really good. Then the second one was a very good set of tennis, I would say.
“Was just one little ball here and there why I lost at the end.”