Mats' Musings: Big test ahead for Djokovic

Editor's note: Former world No.1 and seven-time Grand Slam champion Mats Wilander is writing exclusively for Yahoo! Sports throughout the U.S. Open. The 1988 U.S. Open champ breaks down who's hot and who's not, offering his thoughts courtesy of Lacoste.

It is great to see the smile back on the face of Novak Djokovic but there are still a lot of questions he has to answer if he is going to be grinning at the end of this weekend.

Djokovic's victory over Fernando Verdasco on Wednesday put him into the semifinals of the U.S. Open for the third straight year, yet rarely has there been less confidence in the Serb's ability to progress further.

Serious doubts remain about the world No. 3 in terms of his mentality and, just as important, his fitness.

He does look stronger than he was a year ago, but we must remember that this is a player who has withdrawn three times from matches in his Grand Slam career.

As a player who never retired from a match in my life I find it pretty hard to understand how someone can get into that position in such big tournaments.

For the last couple of years Djokovic has looked unhappy. He stopped doing his impressions of other players and he got on the wrong side of the U.S. Open by being involved in a dispute with Andy Roddick last year.

His fun and games with John McEnroe a few nights ago was a good idea and he is Mr. Popular again now, so the crowd will not be against him when he plays Roger Federer in the semis.

He has beaten Federer at this stage of a major before, on his way to winning the Australian Open in 2008.

The problem is that Federer looks so strong just now and it is difficult to see any player other than Rafael Nadal challenging him over five sets.

Djokovic has a big-time game but will need to produce the match of his life if he is to go any further.

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