The five most exciting clashes to watch

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Sprints (100/200/4x100 relay)

United States vs Jamaica

Jamaica always produced fine sprinters. Trouble was, some like Donovan Bailey ended up in Canada while others who stayed, like Don Quarrie, did enjoy success in the 1976 200 final, but there were never enough to challenge the US undoubted supremacy.

Since the advent of Usain Bolt, the pendulum has swung the Caribbean island's way with individual and relay success sweeping aside the Americans with disdain.

Coming into the Games, though, the signs are that it is the Americans who are on the up.

Bolt has an unspecified injury, as well as suspect form after losing to Yohan Blake in the national 100 and 200 trials, Asafa Powell's frailty coming into major championships has flared again with a groin injury, leaving just world 100 champ Blake looking properly set to challenge the Americans.

However, the Americans too have question marks hanging over them. Will Tyson Gay, as fragile as Powell, hold up through the pressure and intensity of the heats, and can Justin Gatlin pull off one of the most remarkable comebacks of all time in returning from a drugs ban to add a second Olympic 100 gold to the one he won in 2004 in Athens? "People just want to see how far the ride takes me," Gatlin said.

"I don't think anybody expected me to get this far. Just being able to come back and compete is a victory in itself."

110m hurdles

Liu Xiang (CHN) vs Dayron Robles (CUB)

The Americans such as Aries Merritt will have their say in this event, but Liu and Robles are the finest protagonists in the event over the past eight years, with both having something to prove.

They already have Olympic gold - though Liu would have readily swapped his 2004 title for one in Beijing in 2008, where he failed to even get over one hurdle as injury robbed him of glory in front of his fans.

Robles too would probably have preferred Liu to be in the field where he won gold as he has yet to best him in a major final. He thought he had in last year's world final, only to be disqualified for touching Liu's arm twice in the final stages of the race.

Liu looked to be returning to his best form with two impressive wins outdoors this season, but he got the worst possible 29th birthday present on Friday when he pulled out of the London Diamond League event with a bad back.

Robles has not been seen at all with his return from a lead leg injury being constantly announced before not appearing. Not the best of preparations for either of them, but they have the championship mentality and an Olympics should see the best of them.

Dai Greene (GBR) vs Javier Culsom (PUR)

You can also throw 2000 and 2008 Olympic champion Angelo Taylor into the mix, but these two are the ones who have dominated - Greene at championships over the past two years and Culsom this season. Greene, who is bidding to add the Olympic gold to make it a full house of continental titles, underwent a knee operation in the winter but has slowly run himself back into form.

Culsom has been unbeatable this season and firmly put Greene and Taylor in their places at the London Diamond League meet as he ran the fastest time of the year. Culsom, who will be 28 on July 25, is keen to turn two successive world silvers into Olympic gold and believes he is a stronger man than when he lost to Greene in the world final in Daegu last year.

"I've done more weight training, and I run 800m in training. That's why I finish strongly," he said.

Greene, 26, is not worried in the least by Culsom's form.

"I'm only a couple of meters off him and I'm on an incline so no doubt I'll be at his shoulder when the Olympics come around."

Heptathlon

Jessica Ennis (GBR) vs Tatyana Chernova (RUS)

Most athletes just have the pressure on them for one event but the pressure on Ennis - the host nation's pin-up girl and one of the promotional faces of the Games - will last for seven events over two days as the 26-year-old former outdoor world champion bids to deliver the gold she has yearned for since London won the right to host the Games in 2005.

Missing Beijing because of injury has only heightened her determination to win the title.

However, to do so she will have to avenge the loss of her world crown to 24-year-old Chernova last year. Ennis suggested she was in the right form when she blitzed 2000 Olympic champion Denise Lewis's 12-year-old British heptathlon record in May in Gotzis and had Chernova back in second.

"This gives me the self-belief going ahead," said Ennis, and the whole of Britain will hope so. Chernova will be bidding to emulate her mother, who won 4x400 relay gold for the then Soviet Union in 1980.

Shelly-Ann Fraser-Price (JAM) vs Carmelita Jeter (USA)

Fraser-Price bids to ensure her Olympic crown doesn't go the same way as her world title last year and fall into the hands of the late developing 32-year-old Jeter.

Fraser-Price, 25, was completely off form in Daegu, finishing fourth, though her mind may have been more on her recent marriage and also a bit of rustiness, having served a six-month suspension for a banned substance being in medication for toothache.

Jeter has been phenomenal after her belated arrival on the scene - she says because of longstanding hamstring issues remedied by deep tissue massage - and is the second fastest woman of all time, only behind the late Florence Griffith-Joyner.