Review system challenges top men at Australian Open

The challenge system is proving, well, challenging for the top men at the Australian Open.

No. 4-ranked Andy Murray asked for five line calls to be reviewed by video technology in his quarterfinal against Kei Nishikori on Wednesday and failed on every one. No. 2 Rafael Nadal and No. 6 Tomas Berdych challenged a combined 20 calls the previous evening and got eight right.

Nadal was infuriated at 5-5 in the first-set tiebreaker when he tried to challenge a call after he had returned a shot, only to be told by umpire Carlos Bernardes he was too late.

The Spaniard later explained that he was more upset that Bernardes hadn't overruled the call - television replays showed the ball was clearly long.

Nadal believes the challenge system, which allows players three incorrect challenges per set, has discouraged umpires from intervening on line calls.

"They are there to make something, not just to call 15-all or 15-30 all the match," he said. "That's the only thing I'm unhappy about."

He was a bit more pointed during the match when he said to Bernardes in Spanish: "You're not here as a spectator."

Nadal isn't doing too badly on his challenges. He has been right with seven of 16 in his five matches so far for a 43.8 percent success rate.

Murray has managed to get five right from 22 for a miserable 22.7 percent, with Roger Federer - who isn't too keen on the system - on three from eight. Of the men's top four, Novak Djokovic is the most accurate with an accuracy rate of exactly 50 percent.

Aussie teenager Bernard Tomic has been one of the least accurate at Melbourne Park. In his four matches, he challenged 24 times and only had the call overturned on 3 occasions.