One of the signature features of Davis Cup is that it sometimes offers an express-track journey from goat to hero for either of a nation’s two singles players. And I’m not talking about the Roger Federer/Michael Jordan kind of G.O.A.T., but the Jackie Smith/Roberto Baggio type.

That’s because doubles is often the ray-of-hope match in a tie that can breathe new life into a squad that fell behind, 2-1, after the first two days of play—as the U.S. is over in Spain. Or, in less dramatic circumstances, a doubles win can put a player who blew his first assignment in a position to clinch the tie, in either a fourth or fifth match.  
You win the doubles when you’re down 0-2 and suddenly you are just one lousy, stinking, any-given-day match away from being dead even, into the unpredictable territory of the fifth, sudden-death rubber. And the same win at 1-1 puts you in the driver’s seat on the final day.  
When a tie is still live going into the third day, a singles player who failed to deliver as expected on the first day is offered instant redemption, no coupon or code needed. This is the situation John Isner of the U.S. finds himself in on the eve of his fourth rubber (for you Davis Cup novices, that’s a synonym for “match”) meeting with Spain’s No. 1, David Ferrer.  
It’s a mite harsh to accuse Isner of having choked in his first-day meeting with Nicolas Almagro (it was the second rubber of the day, right after Ferrer took apart Sam Querrey); after all, at No. 10, Almagro is ranked two spots higher than Isner, and the match was on Almagro’s favorite surface, clay, and at home.  
Nevertheless, Isner once again appeared to run out of gas and lost the 10th five set match of his career. He has just four wins in five-setters, which is troubling when you take into account that his ability to hold serve—and the trouble he has breaking serve—have already demonstrated that you can count on him to become embroiled in marathons. After more than four hours on court Friday, Isner yielded a 7-5 fifth set to Almagro in a way that can only be described as anti-climactic—with a flurry of avert-your-eyes volley errors.  
It was an especially disappointing loss because it pricked the Davis Cup balloon the U.S. had unexpectedly inflated with those stunning wins over Switzerland and France in the first two rounds of the competition—ties in which Isner had posted terrific wins over Roger Federer, Jo-Wilfried Tsonga, and Gilles Simon. Both were on clay, and on the road.  
In Davis Cup, though, the critical battle between the No. 1 players is pre-ordained as the fourth rubber, or the first match on Sunday. With the Bryan brothers having won the doubles to avert a sweep by Spain, an Isner win over Ferrer would be monumental. It would certainly restore what luster he lost on Friday.  
“Tomorrow, obviously, is a little bit different with one versus one and two versus two,” Courier told the press after the doubles. “Ferrer is such a warrior and a great competitor and John’s going to have to play better than he did yesterday to stand a chance. He’s a little bit more physically taxed than David is for sure.  
“Sam I think will have a chance. If it gets to a fifth match a lot of the pressure actually shifts in the other direction. Momentum is important and it shifts quickly in Davis Cup.”  
Momentum will not have much of a chance to shift down along the Rio Plata in Argentina, where the Czech Republic leads 2-1. But until the late-breaking news that Juan Martin del Potro will not contest that critical fourth-rubber clash of the No. 1s, it looked as if Juan Monaco might be offered a chance to make up for waste of a big lead, at home, on his best surface, against Tomas Berdych.  
Monaco, the ATP No. 11, had a two-sets-to-one lead on Berdych in the second rubber of the tie, following del Potro’s clinical win over aging and injury-plagued Radek Stepanek. But Monaco lost, and when the Czechs won the doubles (with their singles players pairing up to beat the Argentine team of Carlos Berlocq and Eduardo Schwank), they were facing basketball’s familiar “two chances to make one.”  
A win by del Potro over Berdych in the fourth rubber would have set up Monaco to meet Stepanek in the decisive fifth and final match, the “hero” match. But Carlos Berlocq will now face Berdych in that critical opener. It’s hard to imagine Berlocq, No. 45 and playing in his very first Davis Cup tie, felling the tall Czech. So Monaco may be denied a chance to redeem himself in a winnable fifth rubber that would send the triumphant team into the final.  
If Berlocq does pull off the win, it will go down in Davis Cup history as one of the great upsets, especially if Argentina advances to the final with a win by Monaco. But the pressure on Monaco to close the deal will be enormous, and the occasion that much more compelling.  
It’s unlikely that this scenario will come to pass, just as it’s a reach to envision back-to-back wins by Isner and Querrey in Spain. Tennis is a game of almosts and “if onlys,” and as badly as Isner and Monaco might want another chance to redeem themselves, it looks an awful lot like the Czechs will host Spain in the Davis Cup final.