Roger Federer was unhappy with the pace of the indoor court after defeat by David Nalbandian at the Paris Masters.
Federer lost to Nalbandian for the second time in 11 days
In-form Nalbandian took a thrilling match 6-4 7-6 (7-3) for his second victory over the world number one in less than two weeks.
Federer praised the Argentine's "phenomenal" backhand but admitted his frustration with the playing surface.
"I wish conditions were a bit faster because indoors is supposed to be fast," he said. "Not here, I guess."
Nalbandian said: "Well, it's different. Over there (in Madrid) it's a little faster."
He's got a phenomenal backhand and can dictate play from both sides
World number 21 Nalbandian beat Federer at the Madrid Masters 11 days ago and made a superb start in Paris, opening up a 5-2 lead as he dictated from the baseline.
Federer battled on and saved five set points, recovering one break, before Nalbandian finally sealed the set with a cross-court forehand.
A straight-sets win looked on the cards when the Argentine served for the match at 5-4 in the second but Federer showed his class with a swift break back.
But it was only a momentary stay of execution as Nalbandian swept through the tie-break, converting his second match point to take his career record against the Swiss to 8-8.
"Federer's the number one in the world," said Nalbandian. "You always have to be at 100%. I was, just like in Madrid."
Federer admitted: "It's not easy being aggressive against him when you're three metres behind the baseline.
"He's got a phenomenal backhand and can dictate play from both sides."
Nalbandian goes on to face Spain's David Ferrer in the quarter-finals.