LONDON (Reuters) - Michael Phelps beat his old rival Milorad Cavic at the London Olympics on Thursday to book his place in the semi-finals of the men's 100 meters butterfly, his last individual event before he retires.

Phelps famously beat Cavic by a fingernail in Beijing four years ago to win the seventh of his eight gold medals and the American again had the edge when they squared off in the heats.

Phelps stormed home on the last lap to win in a time of 51.54 seconds to qualify second fastest overall. Cavic was fifth overall after touching the wall in 51.90.

The top 16 swimmers advanced to Thursday evening's semi-finals, from which the eight best will go through to Friday's final.

"I just wanted to come in and have a solid morning," Phelps said. "That was fine. I was able to finish pretty well."

Friday's final looms as a poignant moment for Phelps, who won the 100 butterfly gold at the last two Olympics.

No male swimmer has ever won the same individual event at three Olympics and that could be his last chance if he is beaten in Thursday night's 200 individual medley final.

While Cavic has been his main rival for the past four years, South Africa's Chad le Clos has surfaced as his biggest danger in London.

Le Clos beat Phelps with a desperate late lunge to win the 200 butterfly final on Tuesday and qualified fastest for the 100 in 51.54.