Novak Djokovic's win over David Goffin was overshadowed by a clash with the umpire in London. Later, Milos Raonic marched into the last four with a convincing victory.
Novak Djokovic called for officials to show more respect after the world number two was warned for taking too long between points in his ATP Tour Finals win over David Goffin on Thursday.
Djokovic, who dropped just three games on his way to a 6-1 6-2 win in just 70 minutes, was given a time violation by umpire Fergus Murphy when he took a lengthy pause after losing a long rally in the first set.
The Serbian responded by having an increasingly animated discussion with Murphy at the end of the game, which included Djokovic appearing to say the official "had no clue about the game."
"First of all, I accept and I know that I'm one of the players that takes the most time. There is no doubt. I'm not running away from that," Djokovic said.
"Every time that I'm late, if I get a so-called soft warning or pre-warning, I'll accept it and I won't say a word.
"But I think it's fair, correct and respectful towards the player and to the game if you go over the first time, let's say, over the limit, that you at least get a heads up.
"That's all I'm asking for, to be honest. I know that the rules are strict. But there should be a kind of a feel, a sense for the game.
"It was the fourth game of the match, after a long point, the first time I went over, a few seconds, he gives me a warning.
"I didn't think that was supported by the right facts. That's why I wanted to have a conversation and understand why."
Raonic glides past Thiem
Milos Raonic marched into his first World Tour Finals semifinal with a commanding 7-6, 6-3 victory over Dominic Thiem.
Thiem's serve caused the world number four some problems, but the hard-hitting Canadian came through a tight first set on a tie break.
Raonic was too strong for the Austrian in the second, controlling the set with some unplayable forehands before clinching victory.
Raonic joins Djokovic in the last four, and they will be joined by two from Andy Murray, Stan Wawrinka or Kei Nishikori, who complete their round robin matches on Friday.
mds/pfd (AFP, AP)