ROME: Novak Djokovic made the most of a rain break to transform his play and beat Roger Federer 4-6, 6-3, 6-3 in a thriller on Saturday to reach the final of the ATP Rome Masters.

The world number three and reigning champion here was trailing by a set and a break when the heavens opened to give him some much needed respite.

After a little more than an hour off court, Djokovic came back rejuvenated while Federer's game slowly crumbled.

The Swiss master hadn't even faced a single break point before the rain but was broken five times after it as his radar gradually broke down.

The match had started in bright sunshine, during which time Federer had been at his dominant best and was in complete control.

Djokovic was regularly forced to save break points and at 4-4 Federer finally managed to break serve at the sixth attempt in the set - although in controversial fashion.

As Djokovic was about to hit a second serve at break point, a whistle from the crowd put him off.

That seemed to rattle the 21-year-old Serbian and in the ensuing rally he hit a wild forehand long to gift Federer the break.

Djokovic then glared long and hard into the crowd from where the whistle had come while fans around the stadium chanted "out, out, out!"

Federer served out the set and then broke in the opening game of the second as Djokovic netted a backhand.

The number two seed led 2-1 when the rains came and held his first service game after them.

He had a chance for a double break and a 4-1 lead but after Djokovic saved that and held, the winds changed completely.

The Serbian broke back when Federer sent a backhand slice drifting over the baseline and he broke again for a 5-3 lead when Federer shanked a backhand off the frame.

Djokovic served out the set to love and looked to be in the ascendancy.

However, Federer regrouped and drew first blood in the decider when he broke for a 3-1 lead after a string of Djokovic errors including a nervous wayward forehand on break point.

But Federer then made three unforced errors to give the Serbian a 40-0 lead and a crosscourt forehand winner secured a break back.

After that Federer's game simply crumbled, particularly his backhand. He lost his next service game with a netted backhand slice and then threw away his final service game with another backhand error into the tramlines.

It was the first time Djokovic had beaten Federer on clay. The Serbian needs to retain his title here or he will lose his world number three ranking to Britain's Andy Murray.

He will play either world number one Rafael Nadal or 12th seed Fernando Gonzalez in the final.