In the most dramatic of sprint semi-finals, Anna Meares and Victoria Pendleton clashed, crashed and faced relegations before a photo finish split the pair in a night that ended in tears for the two Olympic Champions.

Their sprint semi-final showdown at the track world championships in Melbourne even exceeded the intense pre-championships hype surrounding the pair.

Pendleton crashed in the first heat, Meares was relegated after winning the second and the pair received official warnings before the British star won the decider in a photo finish by barely a tyre width.

2004 Olympic Champion Meares said she thought she had won the second heat of her semi-final against reigning Olympic Champion Pendleton.

"Unfortunately I gave her a second chance and she took full advantage," Meares said.

"It was really disappointing to feel I had progressed through to the final and then I was so agonisingly close in the third round.

"It was difficult for me to pick myself up emotionally to race as hard as I could for the bronze medal and I did and I'm proud of that."

Meares, the defending world champion, had broken the world record for the flying 200m in qualifying and then sailed through the opening rounds of match racing before the clash with Pendleton.

To cap an emotional night, Meares overcame Lyubov Shulikato (UKR) in the bronze medal race.

"As disappointing as it is, it's a world championship bronze medal and I think it will hold me well for London," said Meares.

"It's probably the most difficult race I've had to win - that bronze medal."

Meares was not afraid to hide her emotions and shed a tear after embracing her parents in the stands post-race.

Asked about the bigger picture of not winning the team pursuit (with McCulloch) and the women's sprint ahead of the Olympics, Meares said: "Up close and in the moment, yes, losing those events - yes, it hurts.

"It's a bit difficult to see the big picture when the big picture is right in front of your face."

There was more drama in Pendleton's gold-medal ride against Lithuanian Simona Krupeckaite.

After winning a tight second heat to level the score, Krupeckaite was relegated for not holding her line in the final sprint and that gave Pendleton the gold medal.

Pendleton fell to the ground in disbelief and was overcome with emotion, the tears flowing.

Pendleton and Meares have their last race at the worlds on Saturday in the women's keirin, another Olympic event.

The British ace has now won nine world titles across the three sprint events.

"This is probably the most significant and probably one of the most emotional world titles," Pendleton said. 

It was the ultimate preview ahead of the London Olympics.

AOC with AAP