LONDON -- Novak Djokovic can make tennis look easy, and he can make opponents look bad.
The defending champion did a bit of both on Centre Court at Wimbledon on Wednesday, beating Denis Kudla of the United States 6-3, 6-2, 6-2 in the second round.
Despite the straightforward score, however, Djokovic was short of perfection as he moves toward a fifth title at the All England Club. His serve was broken twice, once in each of the first two sets.
"There were some moments in the match where maybe I could have done better," the top-seeded Serb said. "Dropped a couple of times my serve."
The service hiccups didn't really matter, though, because Djokovic made up for it by breaking Kudla multiple times in each set -- seven in total.
Djokovic will next face Hubert Hurkacz on Friday,
Djokovic beat Anderson in the 2018 final for his fourth Wimbledon title, and they could face each other in the semifinals of this year's tournament.
The 63rd-ranked Opelka, who is making his debut at Wimbledon, earned the only service break of the fifth set in the last game, helped by four groundstroke errors from Wawrinka, who was seeded 22nd.
"At first I had a lot of success serving and volleying, so I kept with that," Opelka said. "And then as he kind of picked up on what I was doing and started reading my serve a little bit, it was more difficult for me to win points at the net. So I had to play, played a lot of tennis on the baseline today."
This was only the sixth Grand Slam match of Opelka's career. Opelka, 21, defeated John Isner in the first round of the Australian Open in January.
The 6-foot-11 Opelka used his serve to get things done against Wawrinka. He finished with 23 aces, with several serves topping 140 mph. One, at 142 mph, tied with Alexander Zverev for the fastest of the tournament so far.
Eight of Opelka's aces came in the last set, when he saved both break points he faced. In all, he fended off 10 of Wawrinka's 12 break chances.
"I had some chances," Wawrinka said. "I start a little bit slow. I was a bit hesitating, not really moving well enough. ... At the end he went for it. He went bigger than me and he deserved to win."
It was another upset in a tournament filled with early surprises. On Monday, both No. 6 Zverev and No. 7 Stefanos Tsitsipas were eliminated. No. 5 Dominic Thiem, a French Open finalist the past two years, followed them out of the tournament on Tuesday.
Opelka becomes the first American man since Justin Gimelstob in 1997 to knock out a previous major title winner in his Wimbledon main draw debut.
Wimbledon is the only major tournament Wawrinka hasn't won. He has lost in either the first or second round at the All England Club in each of the past four years since reaching the quarterfinals in 2014 and 2015.
Meanwhile, former finalist Milos Raonic is through to the third round of Wimbledon for the sixth year in a row.
Raonic beat Robin Haase 7-6 (1), 7-5, 7-6 (4) to set up a meeting with Opelka.
Raonic needed treatment on his left ankle and Achilles when trailing 5-4 in the third set, but he finished off the win by converting his first match point in the tiebreaker. The Canadian was broken only once in the match as Haase took a 6-5 lead in the first set, but Raonic broke right back. Raonic lost the 2016 final to Andy Murray.
With Murray not playing singles, Edmund was Britain's highest-seeded player in the men's draw at No. 30. However, he appeared to aggravate a right knee injury when he slipped on the grass near the end of the third set and needed a medical timeout.
Verdasco reached the Wimbledon quarterfinals in 2013 but had lost in the first round in each of the last three years.
At 40, Karlovic became the oldest man since 1975 to win at Wimbledon by advancing to the second round. The Croatian finished with 38 aces but only forced one break point in the match, which he couldn't convert.
On Monday, Auger-Aliassime became the first male player born in the 2000s to win a Grand Slam match.
Widely viewed as one of the most promising talents on the men's tour, Auger-Aliassime missed the French Open with an adductor injury but reached the final on grass at Stuttgart and the semifinals at Queen's in the lead-up to Wimbledon.
The Associated Press contributed to this report.