Novak Djokovic has hired Thomas Muster's old trainer as his new fitness coach.

The third-ranked Serb started working with Gebhard Phil-Gritsch after losing to Rafael Nadal in last week's Monte Carlo Masters final.

The move comes at a key point in Djokovic's season. He is the defending champion at the Rome Masters, which starts Monday, then plays at home in Belgrade, followed by the Madrid Masters and the French Open, which starts May 25.

"(Fitness) is what I'm going to need a lot of in the next period, which is very difficult," Djokovic said Sunday. "I play all three weeks and then we have the French Open, and we all know the transition between the clay and grass is very short. It's going to be a very exhausting couple of months."

Muster won three titles on the clay at the Foro Italico in the 1990s and won the French Open in 1995.

Djokovic was the only player to take a set from Nadal in Monte Carlo, but he appeared to grow weary in the third set before getting beat by the Spaniard, 6-3, 2-6, 6-1.

"I've been always trying to be a better player and to improve myself in any aspect, so as soon as I felt that I cannot improve or learn anymore with a certain member of my team, no hard feelings, but I have to change some things," Djokovic said. "I was thinking about it for the last couple of months."

Muster was renowned for his fitness and clay-court prowess, and Djokovic labeled his first week with Phil-Gritsch as painful.

"It's a new program, a new system, a new person, so obviously a lot of innovations, but of course we didn't take it from the start, from zero, because it's too much risk to change everything right away. But I think we have a great base, and he has a good knowledge that we can use," Djokovic said. "We are working a lot on endurance and strength in the legs, because clay requires a lot of endurance."

Djokovic retired from his quarterfinal match at the Australian Open against Andy Roddick due to illness brought on by the heat. He also changed his racket this year, going from Wilson to Head. He has reached the final in his last two events, losing to Andy Murray at the Sony Ericsson Open before getting beaten by Nadal last week.

"I think things are coming together for me," Djokovic said. "I'm (improved) fitness-wise and tennis-wise, and hopefully I can reach the most important level for the most important events in this period of the year, which are Roland Garros and Wimbledon. I'm back on the right path, I just need to keep it up."