* Djokovic, Nadal gain walkovers
* Both advance to final on free passes
* Djokovic will play for fourth Miami title (Changes slug, adds Berdych withdrawal)
MIAMI, March 28 (Reuters) - World number one Rafa Nadal and number two Novak Djokovic advanced to the final of the Sony Open in shocking fashion, receiving walkovers after their opponents pulled out of the tournament within hours of each other on Friday.
Djokovic was first gifted a spot in Sunday's final when Japan's Kei Nishikori withdrew with a groin injury.
A few hours later the tournament was rocked again when seventh seeded Czech Tomas Berdych announced he was withdrawing due to gastroenteritis, handing Nadal a free pass.
Thousands of tennis fans had arrived at the Crandon Park complex expecting a day of pulsating action but instead were left with a two doubles matches.
"Sorry for Kei. Sorry for Thomas. Sorry for the tournament. Especially sorry for the fans," Nadal said on his Twitter account after the double walkover.
Nadal advanced to the final for a fourth time where he will try for his first title on the Miami hardcourts against three-time champion Djokovic.
Nishikori had reached the last four after registering a pair of three-set upsets, defeating Spanish fourth seed David Ferrer in fourth round followed by a quarter-final win over Swiss 17-times grand slam winner Roger Federer.
The ATP Tour announcement Nishikori and pulled out just before the 20th seed was about to take the court.
"It's really sad, of course, semi-final in a big tournament," Nishikori told reporters. "Was really playing well and beating (Grigor) Dimitrov, David (Ferrer) and Roger (Federer). I was really excited to play here the semis.
"But unfortunately I couldn't move side-to-side. Just tried to warm up today, but I couldn't move."
Nishikori said that the groin had been bothering him for sometime but flared up during his match against Federer that ran late into an unusually cool evening in South Florida.
The Japanese number one also retired during the Delray Beach tournament in February for similar reasons.
"I really felt the last match against Roger, the quarterfinal," said Nishikori. "I had it before Indian Wells, and I hurt it in Delray. It wasn't 100 percent yet."
Nishikori will return to Japan where he will see doctors to determine the extent of the injury that could keep him out of Japan's Davis Cup tie next month against the Czech Republic.
"I don't know if I can play or not but I will try," said Nishikori. "It's not going to be like surgery bad, but it's still...I'm feeling it, so I have got to see a doctor right now."
Berdych said the sickness had affected him for most of the day.
"I just woke up with a pain in my stomach, just went for toilet, and got really strong diarrhea," he said. "Since then, it starts to go on and on. More time, diarrhea and then also throwing up and stuff like that.
"I lost so much of the liquid and all the possible energy I could have," he added.
"Came here. Tried to do as much as I could, see the doctors, receiving the IV, and basically not with good results."
The walkover was the second of the tournament for Djokovic, the Serb also receiving a free pass in the third round when Florian Mayer pulled out, also with a groin injury.
Djokovic should be well-rested for Sunday's final, not having played in four days and contested just three matches the entire tournament.
Friday's walkover was the second of the tournament for the Serb, who also received a free pass in the third round when Florian Mayer pulled out, also with a groin injury.
"It's not the first time it happens to me (a walkover), maybe it is the first time twice in the same tournament but it is the way it is and you kind of have to deal with that mentally," said Djokovic. "I practiced today, 30-40 minutes on centre court, got good rhythm and trying to keep up with what I have been doing.
"I won in Indian Wells, gave me a lot of confidence, played a lot of matches so hopefully it will pay off." (Reporting by Steve Keating in Miami. Editing by Pritha Sarkar and Gene Cherry)