Before the start of the Davis Cup clash between Spain and Argentina, the South American nation were firm favourites to take a 2-0 lead on the opening day. But at the end of the opening day's play, they were level at 1-1.

Argentina's David Nalbandian comfortably defeated David Ferrer 6-3 6-2 6-3 in the opening rubber, before Spain's Feliciano Lopez produced a minor upset to defeat Juan Martin del Potro 4-6 7-6(2) 7-6(4) 6-3.

With the groin injury that Del Potro picked up towards the end of his encounter, the hopes of Argentina taking home their first Davis Cup look more uncertain than ever, now that it has been reported that Del Potro will not be fit enough to take the court for the reverse singles.

Today's doubles rubber should be the decisive factor in this final, where the winning nation should be favoured to lift the trophy on Sunday. Spain should start the match as slight favourites given their better track record in Davis Cup doubles, whereas Nalbandian/Calleri have shown an inconsistent level in the past, most notably in the 2006 final, where the Argentine pair meekly went down to Marat Safin and Dmitry Tursunov in three very one-sided sets.

The day started off brightly for the Argentine team, with Nalbandian, relishing the opportunity of playing for his country, finding his best tennis to dispatch Ferrer comfortably. Nalbandian has openly expressed his burning desire to win the Davis Cup, and he was extremely fired up. In this frame of mind, Nalbandian is tough to beat, showing the sort of killer instinct that is often lacking on the main tour. 

That added purpose and determination in his mindset means that he doesn't give away any more cheap points than necessary, and on the occasion where he throws in a couple of poor errors, he bounces right back and continues to pour the pressure on his opponent. Ferrer had chances, but whenever it felt like the match was building up to a contest, Nalbandian either edged it out to hold onto his serve, or bounced back strongly to break serve in the following game.

Ferrer has been struggling with his form of late, and that showed in his patchy performance. He doesn't strike me as the kind of player who has the inner confidence to rise to the occasion, when he doesn't have the match wins under his belt. On the other hand, he is a battler and once he gets his teeth into a match, he can be dangerous.

Nalbandian had the superior shot-making ability, often catching Ferrer off-guard with changes of pace down the line and angled shots particularly on the forehand. The key for Nalbandian is to attack selectively, but slow down the pace when in a defensive position unless he is attempting a winning shot. In particular, he was winning a lot of points on the return of serve, nailing that forehand crosscourt when receiving the wide swinging serve on the deuce court.

It was up to Lopez to keep Spain's hopes alive, and he delivered in abundance. Sometimes Lopez can be overly reliant on his own serve, and can be prone to making awful errors, but against Del Potro, he put in one of the most consistently effective performances I've seen from him.

Lopez started off slowly, being outplayed easily from the back of the court, and he handed the early break of serve to Del Potro with four sloppy errors.

Usually, when players face Lopez, they look to target his major weakness, the backhand, where he is severely lacking an attacking shot and resorts to the slice on most occasions. But this was a match-up that strongly favoured Lopez, because Del Potro has been known to struggle with low slice backhands. He also likes to get into a rhythm from the back of the court, which Lopez does not provide him with.

Del Potro showed a lack of tactical awareness, displaying no particular pattern of play in order to win points.  His accuracy was poor, letting Lopez take more of an aggressive mindset and quickly move up to the centre of the court to put away volleys. He needed to take him out of court better and move the Spaniard around. 

On occasion, I noted him trying to trading crosscourt forehands to Lopez's backhand side, but Lopez promptly ran around it hit a forehand. He attempted a similar pattern of play against Roger Federer in Madrid, and got burned consistently. He needs to learn that you need to either open up the court to find an opponent's weakness or do so with much better accuracy.

Lopez seemed to enjoy the pace of Del Potro's groundstrokes as well as the low bounce on the court which enabled him to shorten his backswings and redirect his shots. His groundstrokes were holding up remarkably well, even in the longer rallies. 

Importantly, he was also executing the lower-difficulty shots well, taking care of the high volleys, put away shots and serving consistently well, which can sometimes be a problem with him. He never buckled under pressure, apart from one slight hiccup early in the third set and kept up a consistently high level to defeat Del Potro, who struggled with a groin injury from midway through the third set.