Rome - Rafael Nadal advanced to the Rome Masters third round on Wednesday when fellow Spaniard Nicolas Almagro was forced to quit with a left knee injury.
Three-time French Open quarter-finalist Almagro - he lost all three to Nadal - was unable to continue after just 24 minutes while trailing his compatriot 3-0 in the first set.
It was a 50th win for Nadal in Rome and he is on a hot streak after sweeping the titles on clay at Monte Carlo, Barcelona and last weekend in Madrid.
"I am happy with almost everything, I am playing well. I won a lot of matches this year," said the fourth seed, who next faces Jack Sock as he builds up to Roland Garros where he will be chasing a 10th title.
"There is always things to improve, but it is not the right time to say I'm not happy with one thing or another.
"Always there is the motivation to keep going and to keep improving something."
Nadal though was distressed to see Almagro injured who he also helped from the court.
"It's difficult to say something because we need to wait a couple of days. It's difficult to see what's going on now just after what happened," he said.
Joining Nadal in the next round was Swiss third seed Stan Wawrinka, who beat Benoit Paire 6-3, 1-6, 6-3 while Canadian fifth seed Milos Raonic ended the Rome run of 39-year-old Tommy Haas, 6-4, 6-3.
There was also a win for Japanese seventh seed Kei Nishikori, with Asia's best player defeating David Ferrer 7-5, 6-2.
Madrid finalist Dominic Thiem started his week with a win over Pablo Cuevas 7-6 (7/4), 6-4. Sock defeated Jiri Vesely 6-4, 3-6, 7-6 (7/1).
In women's play, world number one Angelique Kerber crashed out, the top seed beaten 6-4, 6-0 in the second round by Anett Kontaveit of Estonia.
The German's 56-minute knock-out came two days after she was confirmed at the top of the WTA rankings, displacing Serena Williams after making it to the third round at last week's Madrid Open.
The defeat demonstrated more of the patchy and inconsistent form which the 29-year-old cannot seem to shake off since starting to duel with Williams at the top of the rankings last season.
Second seed Karolina Pliskova beat Lauren Davis 6-1, 6-1, Madrid champion Simona Halep started with a win over Germany's Laura Siegemund, 6-4, 6-4 and eighth seed Elina Svitolina put out Alize Cornet 6-4, 7-6 (13/11).
French Open champion Garbine Muguruza of Spain had to recover to post
a 2-6, 6-2, 6-1 win over Latvian qualifier Jelena Ostapenko.
Results on the fourth day of the Rome Masters on Wednesday (x denotes seeding):
Alexander Zverev (GER x16) bt Viktor Troicki (SRB) 6-3, 6-4
Milos Raonic (CAN x5) bt Tommy Haas (GER) 6-4, 6-3
Stan Wawrinka (SUI x3) bt Benoît Paire (FRA) 6-3, 1-6, 6-3
Dominic Thiem (AUT x8) bt Pablo Cuevas (URU) 7-6 (7/4), 6-4
Sam Querrey (USA) bt Jan-Lennard Struff (GER) 6-4, 6-3
Jack Sock (USA x13) bt Jirí Veselý (CZE) 6-4, 3-6, 7-6 (7/1)
Rafael Nadal (ESP x4) bt Nicolas Almagro (ESP) 3-0 - retired
Kei Nishikori (JPN x7) bt David Ferrer (ESP) 7-5, 6-2
Juan Martín Del Potro (ARG) bt Kyle Edmund (GBR) 7-5, 6-4
Roberto Bautista Agut (ESP) bt Pablo Carreño-Busta (ESP x15) 6-7 (6/8), 6-1, 6-4
Anett Kontaveit (EST) bt Angelique Kerber (GER x1) 6-4, 6-0
Anastasia Pavlyuchenkova (RUS x12) bt Anastasija Sevastova (LAT) 7-6 (7/4), 6-2
Simona Halep (ROM x6) bt Laura Siegemund (GER) 6-4, 6-4
Ekaterina Makarova (RUS) bt Dominika Cibulkova (SVK x4) 1-6, 6-1, 6-3
Kiki Bertens (NED x15) bt Catherine Bellis (USA) 6-4, 6-0
Daria Gavrilova (AUS) bt Caroline Garcia (FRA) 7-5, 3-6, 6-3
Venus Williams (USA x9) bt Lesya Tsurenko (UKR) 6-4, 6-3
Julia Goerges (GER) bt Jelena Jankovic (SRB) 2-6, 7-6 (7/2), 6-1
Garbiñe Muguruza (ESP x3) bt Jelena Ostapenko (LAT) 2-6, 6-2, 6-1
Elina Svitolina (UKR x8) bt Alize Cornet (FRA) 6-4, 7-6 (13/11)
Mona Barthel (GER) bt Qiang Wang (CHN) 6-3, 6-4
Timea Bacsinszky (SUI) bt Barbora Strycova (CZE x14) 6-4, 6-2
Karolína Pliskova (CZE x2) bt Lauren Davis (USA) 6-1, 6-1