Once again, emerging Davis Cup hero Novak Djokovic has been asked to step up for Serbia, as well as for the Davis Cup competition. And once more, Djokovic is answering the call.

If you remember the first round of World Group play, you’ll recall that Djokovic won the Australian Open and immediately flew to Belgium where, in the course of just four days, he overcame jet lag, adjusted to indoor red clay (he’d been on outdoor hard courts) and stepped up in grand style.

Viktor Troicki made Nole’s life easier by overcoming Belgian No. 1 David Goffin in the opening rubber. But Djokovic followed that win with a quick, three-set demolition of Olivier Rochus, and the Serbian doubles team then clinched the tie.

It may not be quite as easy for Djokovic this weekend as he leads Serbia against the USA in Boise, Idaho. He’s coming off back-to-back Masters 1000 events and facing two Americans who can bring the heat on hard courts. On the other hand, the transition by region or surface won’t be as demanding as it was a few months ago, and Djokovic has had reasonable rest — he’ll have had a full week to recover and train following his mid-week loss last week in Miami. He’s the only player in action this weekend who can legitimately complain about having been playing too much tennis before the end of the spring hard-court season.

So let’s take a quick look at the four quarterfinal ties, with the head-to-head record in parentheses:

!Serbia at USA (Serbia leads, 1-0)

You could be forgiven if you feel a jolt of déjà vu when you check out the details of this one. These two teams met for the first (and until now, last) time in 2010. And while that first-round tie was on red clay in Belgrade, it seemed to represent a dramatic transition for the USA — for the first time in ages, neither Andy Roddick, James Blake, nor Mardy Fish represented the USA in singles. Instead, it was the new, emerging American stars, the twin towers: John Isner and Sam Querrey.

A lot has happened since Djokovic clinched that tie with a fourth-rubber win over a game, pleasantly-surprising Isner. Not all of those changes have been good, at least for the USA. The pair of ace machines has yet to establish itself as the new, bankable USA singles team. Both men have had ups and downs.

Querrey hit a wall and he also experienced injuries — he’s only played four Davis Cup matches since then. And while Isner has emerged as a Davis Cup hero (he had wins last  year over Roger Federer, Jo-Wilfried Tsonga, and Gilles Simon — all on clay), he’s struggled in general lately. Querrey has surpassed him to become the top-ranked American at No. 20 (Isner is No. 23).

The Serbian team is almost identical to the one that stopped the Americans at home back in 2010, at least in terms of the singles players. Given the pace of the medium-slow hard court, it’s hard to see either Querrey or Isner scoring the big upset of Djokovic, especially not the way either of the American lads have been playing.

The United States needs to win the doubles (consider it done; the USA has Bob and Mike Bryan) and get two wins over Viktor Troicki. That’s not impossible: Troicki is ranked just No. 44, but he’s played above his head on a number of Davis Cup occasions. It would be a great morale booster for the Americans if they can get this done, but I have my doubts.

!France at Argentina (France leads, 5-0)  This is the sexiest tie of the weekend, between two nations that have had trouble living up to their Davis Cup potential. Argentina is — by far — the richest and most powerful tennis nation that has yet to win the Cup. France has a proud tradition and, lately, breathtaking talent — but it hasn’t won the Cup since 2001. How do you manage to do that with you’ve had the likes of Jo-Wilfried Tsonga, Gael Monfils, Richard Gasquet, Gilles Simon and others at your beck-and-call?

So something has to give this weekend; somebody actually has to be a hero! If Argentina were at full strength, only the customary dysfunction of the squad would prevent the pundits from picking it to win on the outdoor red clay. But Juan Martin del Potro apparently is at odds with captain Martin Jaite, so the baby-blue–and-white will be relying on Juan Monaco and Carlos Berlocq, with ever-present Davis Cup veteran David Nalbandian waiting in the  wings — waiting, you feel, for any chance to jump into the fray.

Monaco seems to have drifted back down to earth after his career year, and is now down to No. 19.  Berlocq is ranked no. 71, and he’ll start the tie with a tough assignment: ATP No. 8 Jo-Wilfried Tsonga. But that makes you wonder — what about Horacio Zeballos, who’s No. 39 and beat Rafael Nadal in the Vina del Mar clay-court final (yes, that Rafael Nadal)? Are the Argentines outsmarting themselves? We’ll see.

In any event, the Argentines copped a break when No. 9 Richard Gasquet, a semifinalist in Miami (l. to Murray), turned his ankle. The bad news for Argentina is that his place has been taken by No. 13 Gilles Simon. While Gasquet is off to a great start this year, Simon has a steadier, more reliable temperament. Add the French bench of Julien Benneteau (up to No. 27 and playing the best tennis of his life) and doubles expert Michael Llodra and you have pity Argentina.

!Italy at Canada (First meeting)

This is just the third World Group tie (ever!) for Canada — but then Italy is in a WG tie for just the third time since 2000. But Italy has a strong history in Davis Cup, and it has something Canada lacks — two solid, crafty singles players in Andreas Seppi and Fabio Fognini. The only Canadian singles player ranked within the top 100 is No. 16 Milos Raonic.  You might be tempted to call this a classic “one-man” team but for one intriguing element — Canada’s doubles genius, 40-year old but still No. 6 Daniel Nestor.

The tie will be on a medium-fast hard court in Vancouver, which will greatly help the Canadians, who will start No. 140 Vasek Pospisil alongside Raonic in singles. A first day upset of No. 18 Seppi by Pospisil isn’t out of the question, but it’s more likely that if Canada pulls this one out it will be because Raonic wins two or possibly even three matches (as a partner with Nestor in doubles).

The Italians play better when they host; let’s see how their two excellent singles stars (Fognini is ranked just 13 places behind Seppi, at No. 31) rise to the challenge posed by hard courts and a pre-emptive server in Vancouver.

!Czech Republic at Kazakhstan (Kazakhstan leads, 1-0)

Kazakhstan is slowly building a reputation as a Davis Cup overachiever; it also made the quarterfinals in its World Group debut in 2011. And guess what? The Kazaks have a 1-0 head-to-head advantage over the defending champion Czechs.

That win, incidentally, was on Czech soil. Mikhail Kukushkin emerged as the hero on that occasion, clinching the fifth, decisive rubber with a tense four-set win over Jan Hajek. But it was Andrey Golubev who did the heavy lifting, with two singles wins including an upset of the Czech’s best player, world No. 6 Tomas Berdych.

The big difference this week is that Berdych is out with a bum shoulder, replaced by Ivo Minar. The Czechs have No. 45 Radek Stepanek, the highest ranking player on either squad (by far) as well as the hero of last year’s dramatic final-round win over Spain. But team captain Jaroslav Navratil has nominated no. 94 Jan Hajek and no. 63 Lukas Rosol for singles. It seems pretty clear that the Czechs are holding 34-year Stepanek out of singles as an insurance policy, should they find themselves desperate for a fourth or fifth rubber win.

The Kazaks are taking the more straightforward approach, with Kukushkin and Golubev penciled in for singles. The doubles bears watching in this one. It’s hard to see Stepanek and Minar losing to Kukushkin and 33-year old Yuriy Schukin. But since the Kazaks have never played together, I expect them to pull a Saturday surprise.

My upset special: Canada over Italy, 3-2.