* Back injury ends Mayer's run at Indian Wells
* Nadal into fourth round after playing just one match (Updates details of quake in paras 12-13)
INDIAN WELLS, California, March 11 (Reuters) - Twice champion Rafa Nadal was gifted a place in the last 16 of the BNP Paribas Open on Monday when Argentina's Leonardo Mayer withdrew before their third-round match with a back injury.
Spaniard Nadal, who said he had been "very scared" after experiencing his first earthquake a few hours earlier, will next face Latvian Ernests Gulbis, who fought back to beat Italy's Andreas Seppi 5-7 6-3 6-4.
"That's bad news, for sure, for the fans and for Mayer, especially," world number five Nadal told reporters at the Indian Wells Tennis Garden. "I talked with him. It seemed like it's nothing very, very bad.
"Just a typical back problem, beginning when he was warming up his serve. The physio says maybe in four days, five days he will be ready. That's the most important thing, that nothing is serious."
Asked when he had heard that Mayer had pulled out of the tournament, left-hander Nadal replied: "I was ready (for the match). I was doing my fingers tape.
"I am so sorry for him especially. I talked with him just one minute, two minutes. He was calm and said no. Sad situation."
Mayer's withdrawal came as a mixed blessing for Nadal who could do with more matches under his belt as he continues his comeback after being sidelined for seven months last year by a lingering left knee injury.
Seeded fifth at Indian Wells where he won the title in 2007 and 2009, the former world number one beat American Ryan Harrison 7-6 6-2 in his opening match here as he competes in his first tournament on a hardcourt surface in almost a year.
HEAVILY ANTICIPATED COMEBACK
Nadal only returned to the ATP circuit last month in South America but his heavily anticipated comeback went surprisingly well as he won twice after reaching the final in three relatively minor claycourt events.
"I gonna go a little bit to the gym, I gonna go practise a little bit more later, but no match," Nadal said of his plans for the rest of the day. "I am not practising a lot."
Nadal won his 11th grand slam singles title at the French Open last June before his season came to an abrupt end two weeks later following a defeat to Czech journeyman Lukas Rosol in the second round at Wimbledon.
Mayer's withdrawal was the second shock of the day for Nadal who had earlier been frightened by a magnitude 4.7 earthquake that rattled the Southern California desert.
The quake struck deep beneath a mountain range shortly before 10 a.m. PDT (1700 GMT) local time about 22 miles south of the nearby resort community of Palm Springs.
"I was very scared," Nadal smiled. "First time in my life. I was on the massage table preparing for my warm-up. I think the massage table moves even more.
"I finish the earthquake, and my legs were like this," he grinned while wobbling his legs. (Reporting by Mark Lamport-Stokes; Editing by Frank Pingue/Peter Rutherford/Amlan Chakraborty)