Rafael Nadal cruised to victory in his first match since clinching a return to world number one while Alexander Zverev's 10-match win streak was halted at the WTA and ATP Cincinnati Masters.
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.
Gasquet hasn't taken a set off the 31-year-old Spaniard in 10 consecutive meetings since doing so at Toronto in 2008.
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.
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 at Wimbledon and the Australian Open in their only prior meetings, it was the greatest triumph of his young career.
"I'm happy it finally happened," Tiafoe said of his first win over a top-10 rival. "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 and again to stay level early in the third.
"In the middle of the second set, I was about 10 percent," Zverev said. "He's too good a player for me to beat him at this stage."
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.
"That's why I was extremely joyful. You appreciate all the hard work you've put in and you want to keep it going."
- Venus, Kerber eliminated -
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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.
Though she struggled with leg cramps, Makarova saved a match point in the tie-breaker but needed eight chances to finally subdue Kerber on a drop volley after two hours 38 minutes, lifting her to 5-0 this season 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.
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