American Coco Vandeweghe knocked off Kristina Mladenovic 7-5, 7-5 in the championship match on Sunday to win the Ricoh Open at The Hague, Netherland for the second time in her career.
The sixth-seeded Vendeweghe had seven aces while outlasting the third-seeded Frenchwoman.
Vandeweghe survived two rain delays en route to the victory.
"It was actually really difficult -- we had three rain delay stops -- so it was really difficult to get yourself back into the match and make sure you're fresh and light and ready to go," Vandeweghe told reporters. "Especially the last rain delay in the first set. I was really trying to get myself moving.
"I got myself in a bit of a pickle in the second set, but I battled my way out of it, and sometimes the name of the game is just battling through adversity and problems you cause for yourself."
Heavy rain forced the postponement and play is slated to resume on Monday.
The eighth-seeded Mahut is looking to defend his title. Seventh-seeded Muller is looking for his first ARP title.
Mercedes Cup postponed
The Mercedes Cup championship match between third-seeded Dominic Thiem and seventh-seeded Philipp Kohlschreiber was postponed due to heavy rain at Stuttgart, Germany.
The two players were in the process of their fourth rain delay when the match was called late in the first set at 6-6. Kohlschreiber of Germany had a 3-2 lead in the tiebreaker over the Austrian when play was halted.
Pliskova earns Aegon Open title
Top-seeded Karolina Pliskova fought off a challenge from American Alison Riske to post a 7-6 (8), 7-5 victory in the championship match of the Aegon Open at Nottingham, England.
Pliskova, from the Czech Republic, won her first title of this season and the fifth of her career.
Halep withdraws from Aegon Classic
World No. 5 Simona Halep announced she is pulling out of this week's Aegon Classic in Birmingham, England due to a left Achilles' tendon injury.
Halep, from Romania, is hoping the rest will leave her fresh for Wimbledon, which begins June 27.
"It's nothing serious but it's a bit sore," Halep said. "I have almost two weeks to recover before Wimbledon so I will take a few days' rest and then some treatment and then start to play again."