SHANGHAI -- John Isner served his 1,000th ace of the season, but it wasn't enough to stop defending champion Novak Djokovic from moving into the Shanghai Masters quarterfinals with a 7-5, 6-3 win Thursday.
Top-seeded Djokovic is on a 24-set winning streak in Asia dating back to last year's Shanghai Masters. He won his inaugural appearance at the Japan Open last week without losing a set.
Isner's nine aces against Djokovic took him to 1,007 for the year, reaching the milestone for the sixth season.
Roger Federer saved five set points in the first set as he scrambled to a 7-6 (7), 6-4 win over 13th-seeded David Goffin. Second-seeded Federer holds a 10-1 winning record against Goffin, including all three matches they have played this year.
Despite the outcome, Federer didn't feel in control of their round-of-16 match.
"I never really felt like I had the upper hand over David today," Federer said. "I thought he took really a lot of good decisions, and he was super fast on his feet today, which I think made it really hard for me to get my freebies and winners off the baseline."
Federer will play fifth-seeded Alexander Zverev of Germany on Friday in the quarterfinals. Zverev beat Andrey Rublev of Russia 6-0, 7-6 (4) in the final match of the night. The German served for the match at 5-3 in the second set but couldn't nail down the victory until the tiebreaker.
Third-seeded Daniil Medvedev continued his quest to reach a sixth consecutive ATP Tour final after beating Canadian qualifier Vasek Pospisil 7-6 (7), 7-5 in their round-of-16 match.
"I start to feel different even if I try to stay the same," said Medvedev, who added that other players "start to talk to you differently."
The 248th-ranked Pospisil lost his bid to become the third qualifier ever to make the Shanghai quarterfinals.
Pospisil failed to capitalize on five set points in the first set, including two chances in the tiebreaker.
Medvedev's frustration got the better of him when Pospisil led 4-2, 40-30 in the first set, which led to him smashing his racket repeatedly into the ground at the baseline.
"I usually don't like to do it, to break the rackets, because it's never beautiful," Medvedev said. "But it's true that sometimes it maybe can help to get some emotions out and get also the stress out."
Medvedev, who reached his first Grand Slam final at the US Open in September, leads the tour this year with 56 victories. He next plays 10th-seeded Fabio Fognini of Italy in the quarterfinals.
"I think [he's] the worst player I can play at the moment," Fognini said of Medvedev. "He's really dangerous."
Fognini beat seventh-seeded Karen Khachanov of Russia 6-3, 7-5. Khachanov won their first career meeting in three sets at the China Open quarterfinals in Beijing.
"I'm happy because I was losing to him last week and I was playing really good also," 10th-seeded Fognini said.
The 11th-seeded Matteo Berrettini of Italy outlasted No. 8 Roberto Bautista Agut of Spain 7-6 (5), 6-4.