Players are unsure why they haven't been playing their best during the ATP Tour Finals this week, but most say it's not because they're tired.
During the round robin portion of the event, 11 of 12 matches have been decided in straight sets on the indoor hard-courts at the O2 Arena.
Kei Nishikori won his opening round robin against an erratic Roger Federer but dropped his next two matches 6-0, 6-1 and then 6-1, 6-4.
"I felt fresh, but I just didn’t feel the ball this week,” said Nishikori. “It could be the court or the ball. I don’t know. But, yeah, I just didn’t play well, this week.”
He did say that during the entire season, his effort was "really good"—the 28-year-old recently reached the semifinals of the US Open, the final at Toyko and Vienna, and quarterfinals at Shanghai and Paris.
Alexander Zverev, who is into the semifinals, said he was fatigued from the long season, but Dominic Thiem said that did not affect his performances. The 25-year-old Austrian played poorly in his first two matches before winning the third.
"I don't think it comes from fatigue. One tricky part of tennis is that every week it's different conditions. It's never the same. We always have to get used to different court, to different balls," he said.
"Kei, for example, he was playing amazing in Vienna and in Paris. Now this week obviously he's not playing that well. For me is similar.
"In general, I think players have some troubles here to play because I didn't see a really good match yet."
Following his defeat to Nishikori, Federer pointed partly to different conditions on the different courts he had been practicing on. Since then, and has been taking the day off in between matches.
"As we realized it made me happy, relaxed, motivated for when I do come on court, that it's working, we kept it," said Federer, who has reached the semifinals. "I think if I can't practice on center court, there's no reason to go on a different court, feel a different type of atmosphere. Here it's great.
"Arenas change, temperatures change. It's just a different atmosphere. Speeds can be different, they vary. Some courts they didn't resurface the way they did center court, didn't do it at the same time, all that stuff.
"But sometimes when you're seeking rhythm indoors, it can be frustrating regardless."
Thiem changed his strings, which is unusual for a player during the tournament. He had reached the semifinals of Paris in his previous event.
"I needed to change something because I felt not bad before this tournament. I was playing good," he said.
All the players at the ATP Tour Finals are in the Top 10 in the rankings.
This Week on Tennis Channel:
ATP Finals (Sun - Sun 11.11 - 11.18) - Novak Djokovic, Roger Federer and the world's best collide in the season Finale. Live coverage begins on Tennis Channel Sunday at 7:00am EST.