You can't doubt Chelsea’s spirit. Sometimes, though, the brain works but the legs don’t. Tomorrow’s semi-final is Chelsea’s eighth game in 26 days and their superstars are not superhuman.

The secret of their ability to win relentlessly lies in their fantastic second-half performances. No team in the history of the Premiership has been better at punishing lapses of concentration.

Take the quarter-final, second leg against Valencia. The tie was in the balance in the second half yet Chelsea smelt blood and went for the jugular, punishing the Spanish team’s caution.

Regardless of how well the first 45 minutes have gone at Stamford Bridge, Liverpool should beware. They cannot let their guard down for a moment. Any time you ease off, Chelsea overhaul you, so Liverpool must keep a high tempo and keep pressurising.

There are chinks in the Chelsea armour, though. Signs of vulnerability are appearing — wear and tear after a gruelling year. Since John Terry returned from his back injury he has not been as commanding on crosses, his leap not as springy.

Opponents have let Chelsea off the hook by not taking their chances; Liverpool will not be so wasteful, especially since they will be rested and relaxed after a couple of strolls against Middlesbrough and Wigan Athletic. And Rafael BenÍtez, the Liverpool manager, is a tactical mastermind in Europe.

If Chelsea do things by halves, Liverpool take it in thirds away from home. The first half-hour, they keep it tight, quieten the crowd. The second, they show more ambition and push for a goal; that done, the last third is about hanging on to what they’ve got and possibly nicking another. It’s less dramatic than Chelsea’s style, but it’s very effective.

There’s a real risk that Chelsea’s tired bodies will betray their determined minds tomorrow. Liverpool are perfectly poised to take advantage.