Nadal, a 15-time Grand Slam champion who captured his 10th French Open crown in June, defeated Richard Gasquet 6-3, 6-4.
Nadal, a 15-time Grand Slam champion who captured his 10th French Open crown in June, defeated Richard Gasquet 6-3, 6-4, improving to 15-0 in ATP matches against the Frenchman.
The 31-year-old Spaniard, who hasn't lost a set to Gasquet in 10 consecutive meetings since Toronto in 2008, said the victory was crucial after last week's second-round Montreal exit.
"It was important to win the first match after what happened last week in Montreal," Nadal said. "I'm happy with the way I played."
Nadal was assured overtaking Britain's injured Andy Murray atop the rankings next week when Roger Federer withdrew Monday from Cincinnati with a back injury. He has not topped the rankings since July 2014.
"It was an important goal of mine to come back to that number with all the things that have happened to me since then," Nadal said. "It's something very important and very difficult. I worked very hard for it. For sure that's going to be an emotional moment for me."
Next for Nadal, who had a first-round bye, is a third-round matchup with 24th-ranked countryman Albert Ramos-Vinolas. Nadal has won their three prior meetings, most recently in April's Monte Carlo final.
Zverev, coming off titles in Montreal and Washington and seeking his sixth of the year, admitted exhaustion was a factor in his 4-6, 6-3, 6-4 loss to US teen wildcard Frances Tiafoe, saying the toll of his extraordinary run caught up with him.
"I'm completely dead right now," Zverev said. "I've been dead for the past two days. I finished the match. I could have easily pulled the plug in the second set."
For 19-year-old Tiafoe, a second-round loser to the 20-year-old German this year at Wimbledon and the Australian Open, it was his greatest triumph, his first over a top-10 opponent.
"I'm happy it finally happened," Tiafoe said. "We're going to be playing a lot over the next 10 or 15 years. I didn't want to start out 0-3 against him."
Tiafoe, who next faces US 14th seed John Isner, broke three times to win the second set.
"In the middle of the second set, I was about 10 percent," Zverev said.
Tiafoe forced two break and match points in the last game. When Zverev sent a forehand long, Tiafoe screamed with delight.
"I couldn't believe it. I was so happy," Tiafoe said. "I played a pretty good match. These are ones that change a career."
On the women's side, top-ranked Czech defending champion Karolina Pliskova defeated Russian lucky loser Natalia Vikhlyantseva 6-2, 6-3, dropping Dane Caroline Wozniacki from the list of rivals who can overtake her for world number one this week.
Romania's second-ranked Simona Halep and Ukraine's fourth-ranked Elina Svitolina -- who ousted compatriot Lesia Tsurenko 6-1, 6-4 -- could still swipe the top spot for the first time in their careers.
Germany's third-ranked Angelique Kerber, who lost the top ranking after Wimbledon, lost her chance to reclaim it by falling to Russia's Ekaterina Makarova 6-4, 1-6, 7-6 (13/11).
"It's a tough lose," Kerber said.
Makarova struggled with leg cramps before winning on her eighth match point chance, improving to 5-0 this year against top-5 foes. She faces US wildcard Sloane Stephens next.
"I hope I'll be ready," Makarova said.
Australian 48th-ranked qualifier Ashleigh Barty upset ninth-ranked American Venus Williams 6-3, 2-6, 6-2.
"It's really exciting to have my first top-10 win," Barty said. "I'm really happy."
Seven-time Grand Slam champion Williams, this year's Wimbledon and Australian Open runner-up, will rest before the US Open.
"I'm going to take a break and have a rest and just go be," Williams said. "I'm just planning on being super rested."