Fifth-seed Bautista Agut, who won the title in 2016, outlasted second-seed del Potro, the 2009 champion, in a match of remarkable highs and lows.
The Spanish player swept through the first set in only 31 minutes as del Potro seemed to struggle in warm conditions on a packed center court. But del Potro took advantage of the only break point of the second set to level the match.
The advantage in the third set went one way then the other until Bautista Agut broke del Potro to lead 6-5, then saved a break point to win the match in 2 hours, 7 minutes.
Later, an emotional Bautista Agut dedicated his victory to his family in Spain.
“It feels wonderful,” he said. “It was an unbelievable match today. I’ve been fighting a lot on the court, and I couldn’t believe I was playing my best tennis.
“Today I felt really good on the court, and I played very good tennis throughout the match. Juan Martin is a great champion, he’s very tough to beat, and I played two amazing games in the last two games of the match.”
Del Potro said he found it difficult to play in humid conditions in the daytime Saturday after playing his previous matches at night.
“When I had to play in the sun it was very tough for me,” del Potro said. “Roberto started very well. He moved so much better than me from the baseline.
“And then I think my game improved a little bit. I couldn’t play any better, but Roberto did well at the end of the match, and I made some mistakes.”
Del Potro had the first break point of the match in Bautista’s opening service game, after a rally lasting 32 shots. He dropped serve in the next game, immediately broke back but lost his serve again in the fourth game to give Bautista a 3-1 lead. Bautista broke again in the sixth game to lead 5-1 and comfortably served out the set in a game including his seventh ace.
The second set was closer, with del Potro much more competitive. He struggled throughout the match to profit from his first serve, but looked dangerous when able to hit his famous forehand while moving into the court.
The only break-point chance came against Bautista in the 10th game and del Potro converted to take the set in 42 minutes.
The third set was similar, with no break-point opportunities until the 11th game when Bautista Agut broke del Potro to go ahead 6-5. He again saved a break point before winning the set in 50 minutes.