Rafael Nadal, Roger Federer among top stars with questions to answer

MIAMI -- Seemingly in control of his opening match at the Miami Open on Saturday, Rafael Nadal fell ill and promptly retired against No. 94 Damir Dzumhur.

It was a disappointment for the fifth-ranked Spaniard, who has always enjoyed playing in front of his large contingent of fans in southern Florida.

Nadal, though, wasn't alone in explaining his short-lived sojourn. Through five days, Roger Federer withdrew with a stomach bug, and Stan Wawrinka and Venus Williams were ousted in their opening matches.

How concerned should we be with these star players moving forward? ESPN analyst and former world No. 4 Brad Gilbert weighed in.

Rafael Nadal

The facts: Nadal hasn't won a title in nearly eight months. While he has struggled against a more compete ATP field, Nadal has conceded his confidence, particularly on his famed forehand, has lacked. He has made headway -- notably reaching the final four at Indian Wells -- but hasn't exuded the fearlessness that led him to 14 Grand Slam titles. If there is a sliver of good news, the long clay season is just more than a week away.

Gilbert's take: I'd be more concerned if he were injured. But Rafa was clearly sick, and these things happen. In the locker room, it circulates. Certainly, it was disappointing coming off a decent showing at Indian Wells. A couple more W's in Miami would have given him more momentum heading into the clay season. He has been struggling for more than a year, but if he can stay healthy, I see Nadal finding answers quickly on the dirt.

Roger Federer

The facts: He hasn't played a tennis match in 59 days. Federer, who fell to Novak Djokovic in the Australian Open semifinals, underwent arthroscopic knee surgery after a freak injury preparing a bath for his twin daughters. It was the first major injury in Federer's career. After making a last-minute decision to play in Miami, he withdrew before the event began with a stomach bug.

Gilbert's take: Same as Rafa. Fed was ready to go and came up sick. Of course, he is coming off a knee injury, but I saw him at Indian Wells practicing without a knee brace, so I have fewer concerns on that front. In Miami, it was a case of bad luck. The question now is how aggressive is Federer going to be with his clay schedule now that he has missed so much time.

Stan Wawrinka

The facts: His 2016 results have been favorable -- winning in Chennai and Dubai. He also played well in Australia, falling to Milos Raonic in a grueling fourth-round five-setter. But he lost a heart-breaking third-round match against David Goffin at Indian Wells before being ousted soundly by Andrey Kuznetsov in Miami. Like Nadal, the onset of the clay circuit should rejuvenate Wawrinka, last season's French Open champ.

Gilbert's take: Check out his career at Indian Wells and Miami. Stan has never fared well at either. It's weird how players can play so consistently well, but have one or two events they just don't like or are not comfortable playing. But Stan has played really well for quite some time. He'll rebound quickly on clay.

Venus Williams

The facts: Venus will be 36 in three months, and is ranked No. 13 in the world. The next highest-ranked 35-year-old is Francesca Schiavone -- who is No. 105. But Williams failed to win a single match in the rankings-heavy Indian Wells-Miami swing. That's not the momentum she wanted heading into the clay season, her least-desirable surface.

Gilbert's take: No doubt she's struggling right now, but we have to remember that Venus played in Australia, went to Hawaii for Fed Cup and then won a tournament in Taiwan. Now she's back in the U.S. and traveling from coast to coast. At some point, that travel is going to catch up to anyone -- never mind someone who is 35. But one thing I did notice at Indian Wells and Miami: She's moving really well and defying what most players her age can do. With that movement, she can still be a threat.