Rafael Nadal, who won his fourth title in Canada a year ago, needed almost two hours on Friday to overcome the Italian veteran who shocked him on clay at Monte Carlo in April.
Top-seeded holder Rafael Nadal suffered through a two-hour rain interruption before dismissing Britain's Daniel Evans 7-6 (8/6), 6-4.
Roger Federer took time to get going and was helped by Lloyd Harris suffering a calf problem midway through the match.
The victory on Sunday moved Rafael Nadal on to 18 Grand Slam titles, just two behind Roger Federer on the all-time list.
Dominic Thiem will take on Rafael Nadal in French Open 2019 final.
The semi-final will resume at midday (1000 GMT) on Saturday, organisers said
Top seed Novak Djokovic won 7-5, 6-2, 6-2 and will face Dominic Thiem on Friday in what will be his 35th Grand Slam semifinal.
Dominic Thiem was booted out of his own press conference so that Roland Garros organisers could accommodate Serena Williams.
Dominic Thiem prevailed in two hours 10 minutes against Roger Federer having saved two match points in a 15-minute second set tie-break.
A day after knocking out 11-time Barcelona winner Rafael Nadal in the semi-finals, Dominic Thiem claimed his second title in four events.
Dominic Thiem joins Novak Djokovic as the only player to have defeated Rafael Nadal four times on clay.
Rafael Nadal showed no sign of the knee problems and dominated the match completely.
For Rafael Nadal, it's all about his troublesome left knee while for Novak Djokovic, the major goal will be to emerge from a minor slump.
Dominic Thiem denied Roger Federer a record sixth Indian Wells title and returned to his career-high of fourth in the world -- bumping Federer down to fifth.
Dominic Thiem, ranked eighth in the world, had fallen in two prior Masters finals, both in Madrid but he rallied for a third career win over Roger Federer in five meetings.
Roger Federer will battle 25-year-old Dominic Thiem, who is seeking his first title in one of the ATP's prestigious Masters 1000 events.
Playing in Qatar is an established part of Novak Djokovic's warm-up for the Australian Open.
World number three Roger Federer must beat South Africa's Kevin Anderson in his last group match to make the semi-finals.