Tennis has the replay system by which all other sports are measured. It’s quick, easy and almost entirely non-controversial. Conflicts occur occasionally, mostly about the replaying of points, but it’s generally accepted that the tennis Hawkeye system is the most painless replay method in sports. On Thursday night in Cincinnati, Roger Federer and Gael Monfils exposed its biggest flaw.

It happened during their third-round match at the Western & Southern Open. Monfils was down a set, but had a 0-30 lead on Federer’s serve at 3-3 in the second. Federer came to net and Monfils hit a lofted shot that Federer let go, assuming it would land out. The ball seemed to clip the line and Monfils thought he had three break points at 0-40. But as the Frenchman went to the towel, chair umpire Carlos Bernardes announced the score as 15-30. Monfils, seemingly stunned, inquired why.

Bernardes said the ball was out. Monfils immediately looked like he wanted to challenge the call, but Bernardes basically talked him out of it, holding up his fingers to demonstrate how far out the ball was.

Monfils, still confused, responded with the same finger measurement.

Bernades assured him the ball was out by that much and Monfils resumed play at 15-30. A few points later, a Hawkeye replay showed that the ball had, indeed, clipped the line.

Bernardes assured Monfils that the ball was a few inches out, when the ball was a few centimeters in. It was a pivotal call in the game, which Federer went on to win. Despite the bad call, Monfils went on to break Federer in the next service game and took the second set. No harm, no foul, right?

Not quite. Later in the match, Federer loudly complained to Bernardes about another call, in which Federer’s forehand hit the line, but was called out. Bernardes didn’t immediately offer any advice. This set off Federer, who had as big a tantrum as he’ll ever throw. (Which is pretty mild compared to others.) The minor dust-up didn’t prevent Federer from winning the match in three sets.

Later that night, Federer sat with ESPN for an interview. Darren Cahill eventually brought up Monfils’ shot at 0-30 in the second, but before he could finish the question, Federer interrupted.

“Yeah, that was in,” he said, matter-of-factly. “I don’t understand why he didn’t challenge.”

Federer went on to explain that the Monfils point is what caused him to get so upset with Bernardes later in the match.

“The umpire has to be 100% sure to tell you it’s ‘this much out.’ If he’s not sure it’s this much out, you give the ‘you know, it’s close — you should challenge’ [look]. That’s why I was upset. I know he missed Gael’s call. When I hit my forehand on the line, he kept looking [the other direction], because he didn’t want to give me the ‘I’m not sure.'”

Federer’s right. Though it’s on Monfils for not challenging the call, he was goaded into the indecision by Bernardes. Advising players would be fine if the chair was always positive before dispensing such wisdom. Unfortunately, this isn’t always the case, for good reason.

Bernardes clearly thought the ball was out. If he didn’t, he wouldn’t have called it that way. He’s not a chair umpire for big ATP matches because he’s indecisive. He, and his colleagues, need to be completely sure about all the calls they make. Being wishy-washy isn’t an option.

That’s why giving advice should be discouraged. The chair umpire is inherently biased toward the call he’s made. Though it’s sporting to let a player know how wrong he may be with his potential challenge, let them find out on their own.