* Nadal wins third title since ATP return
* Beats Del Potro in absorbing final
* Moves well on left knee (Adds further Nadal quotes, detail)
INDIAN WELLS, California, March 17 (Reuters) - Rafa Nadal added another triumphant chapter to his remarkable comeback story by beating Juan Martin Del Potro in the BNP Paribas Open final on Sunday to win his third title of the year.
The Spanish left-hander was sidelined for seven months last year with a left knee injury but has been in stunning form since he returned, reaching four straight finals and compiling a 17-1 record to start the year.
Nadal had to come from a set and 1-3 down to beat Argentine Del Potro 4-6 6-3 6-4 and take his 22nd ATP Masters title and a third at Indian Wells.
"I'm very, very happy and very emotional," said 11-times grand slam champion Nadal, who flopped to the ground, flat on his back, shaking his fists in celebration after sealing the win over Del Potro.
"A lot of things happened the last seven months, to be back here and to have this very heavy trophy with me is amazing, no? It's impossible to have a better comeback, no? Happy for everything.
"Beating three top-10s, three very important players, to win a title like this is just something unbelievable for me," added the Spaniard, who beat second-ranked Roger Federer in the quarter-finals and sixth-ranked Tomas Berdych in the last four.
It was Nadal's 53rd ATP singles title and his first on a hardcourt surface since Tokyo in 2010.
The Spaniard made an impressive start, holding serve after a marathon first game where he controlled a series of protracted baseline rallies, then breaking Del Potro in the second, though it took him four break points.
The Argentine was again in trouble on serve in the fourth, serving at 15-40 down, but saved two break points before holding with a booming forehand winner down the line.
Nadal surprisingly failed to hold in the fifth after dictating almost every rally, losing the game when 2009 U.S. Open champion Del Potro blasted a backhand crosscourt winner with his service return.
Growing in confidence and continually attacking with his forehand, the towering Argentine again broke Nadal in the ninth when his opponent blasted a forehand long.
Del Potro held serve to take the first set in 54 minutes, racing 40-0 up but squandering his first two set points before finishing off with a backhand volley winner at the net and pumping his right fist in delight.
An out-of-sorts Nadal failed to hold in the first game of the second set, trailing 30-40 after a blistering forehand pass by the Argentine and then netting a forehand to give Del Potro the break.
The next four games went with serve before Nadal, urged on by roars of "Rafa, Rafa, Rafa" that echoed around the Stadium Court, broke Del Potro in the sixth when the Argentine hit a forehand long.
Del Potro was also broken in the eighth when Nadal blasted a forehand winner into an open court and the Spaniard then held serve to level the match at one-set all with a 105 mph ace.
The Argentine survived three breaks points before holding serve in an energy-sapping first game in the final set but he failed to hold in the third as the Spaniard crunched a forehand winner down the line to lead 2-1.
There was no way back for Del Potro, who had upset world number three Andy Murray in the quarter-finals and top-ranked Novak Djokovic in the last four.
He saved three match points after going 0-40 down on serve in the ninth before holding, but Nadal served out for victory in the 10th, needing only one more match point to add a third Indian Wells title to his previous wins in 2007 and 2009.
The Spaniard, who had his left knee taped up throughout the tournament, edged one ahead of Federer with his record 22nd ATP Masters title. It was also the 600th match win of his career.
Nadal will climb one spot to fourth when the world rankings are issued later on Monday. (Reporting by Mark Lamport-Stokes; Editing by Gene Cherry/Peter Rutherford)