But despite the lack of preparation and the physical toll of the illness, Nadal cruised to a 6-3, 6-3 win over Canadian teenager Felix Auger-Aliassime to reach the third round in the Spanish capital. Playing only a few days after falling ill, Nadal lost only four points on his serve in the first set and broke Auger-Aliassime three times in the second.
"I've had this stomach virus for a few days so this was a very important match for me," the second-ranked Spaniard said. "I've been improving but obviously the body remains a bit debilitated for a while. I'm very happy with the victory. It was very important."
Nadal is trying to rebound from consecutive eliminations in the semifinals of both Monte Carlo and Barcelona, which marked his worst start to the clay-court season in four years. "The important thing is to win, you know, especially given what's happened in the last three days," he said. "In general, it's a day to be very satisfied, very happy."
A five-time champion in Madrid, Nadal will next face American Frances Tiafoe, who defeated Philipp Kohlschreiber 6-4, 3-6, 6-3. "He is playing really well," Nadal said of Tiafoe. "He has that drive and energy and it will be a good test for me and an opportunity for me to play a tough match and to keep improving."
The 18-year-old Auger-Aliassime is having his breakthrough season on tour, having moved to 30th in the rankings after reaching the final in Rio and the semifinals in Miami. He converted on his only break opportunity of the match late in the second set, but Nadal broke right back to close out the match. The 17-time Grand Slam champion converted four of his 10 break opportunities, with the last one coming on his sixth-match point of the final game.
The last match of the day was also the last ever on tour for David Ferrer, whose career came to an end with a 6-4, 6-1 loss to defending champion Alexander Zverev. The 37-year-old Spaniard had said before the tournament he wasn't fit enough to keep competing at a high level.
Ferrer finished his 20-year career with 27 singles titles, fifth-best among active players behind the top four of Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray. Ferrer had 733 wins in 1,111 matches, fourth-best among active players.
"I couldn't keep playing at the level that I wanted, but I'm very happy and very proud of my career," Ferrer said. Earlier, Juan Martin Del Potro squandered a match point in a 6-3, 2-6, 7-5 loss to Laslo Djere in his second tournament since injuring his knee in October. Del Potro converted only two of his 11 break opportunities, failing to capitalize on the match point at 5-4 in the final set. The Argentine had last played in Delray Beach in February, losing in the quarterfinals.
Stan Wawrinka defeated Guido Pella 6-3, 6-4 and will face Kei Nishikori, who got past Bolivian qualifier Hugo Dellien 7-5, 7-5. Fabio Fognini beat John Millman 6-2, 6-2, while Gael Monfils edged Marton Fucsovics 1-6, 6-4, 6-2. Estoril Open champion Stefanos Tsitsipas defeated Adrian Mannarino 6-2, 7-5.
On the women's side, top-ranked Naomi Osaka reached the quarterfinals with a straight-set win over Aliaksandra Sasnovich and will next face Belinda Bencic, who dropped only two games in her victory over Kateryna Kozlova.
"I'm at a really good place right now," Osaka said. "I feel like I'm having fun playing tennis again, which is always a good thing for me and I always play well if I have that mentality." Defending champion Petra Kvitova defeated Caroline Garcia 6-3, 6-3 to set up a quarterfinal match against Kiki Bertens.
Sloane Stephens needed three sets to defeat Saisai Zheng, while third-seeded Simona Halep routed Viktoria Kuzmova 6-0, 6-0. "I think everything went very well for me to today," said Halep, a two-time champion in Madrid. "I felt the ball, every single shot."
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