Monfils struggled in the second set after hurting his knee and was broken twice, winning just eight points on his serve.
"Gael outclassed me in the first set, and after he hurt his knee I was nervous because I wasn't sure to expect," Lopez said. "But I hung in well and played some good serve and volley tennis to take the second set. In the decider, he looked to be back to 100 percent on his feet and it was a close match until the tiebreaker where I just seemed to do everything right."
The 29-year-old Robert reached the first ATP Tour final of his career. He changed his style of play after returning to the game six months ago following a long bout of hepatitis, which sidelined him for 15 months.
"I had been all my life playing what I call 'casino tennis,' hitting crazy shots, gambling with big shots and closing my eyes just hoping the ball would go in," the 100th-ranked Robert said. "Last August, when I came back to tennis after the illness, I had a long chat with my coach, Ronan Lafaix, who persuaded me to do things differently.
"He urged me to start focusing on one thing at a time, to concentrate harder than ever before and develop a precision game."