Wladimir Klitschko had to go the distance to defend his WBA, IBF and WBO heavyweight belts by decision against Mariusz Wach of Poland on Saturday.
The 2.02-meter (6-foot-7 1/2) Wach was the tallest opponent Klitschko faced in his 16-year professional career but offered little threat against the faster and 2-kilo (4-pound) lighter Ukrainian.
The judges scored it 120-107, 120-107 and 119-109 to Klitschko, who was troubled only at the end of the fifth round, when Wach surprised him with a right hook before a flurry of punches left the defending champion on the ropes.
''He caught me in the fifth, but then I slipped,'' said Klitschko, who was left with a small cut under his left eye and must have been relieved to hear the bell.
Klitschko left the 32-year-old Wach wobbling in the seventh with a devastating right blow followed by a series of combinations. Wach somehow hung on.
The two-time Polish champion was clearly in trouble in the next, when Klitschko was delivering punches at will and it seemed only a matter of time before he ended it.
Wach displayed incredible stamina to continue, but his only hope was for a lucky punch that never came.
''That was the hardest fight of my career,'' said Wach, who dropped to 27-1 (15 KOs) after his first loss. ''I want to apologize to everyone for not living up to expectations. I really wanted to win. Thank you so much to my family.''
The 36-year-old Klitschko said Wach ''boxed brilliantly.''
Klitschko improved to 59-3 (51 KOs) after his first bout since the death of his longtime trainer and friend Emanuel Steward.
''I want to remember one man, who can't be here today unfortunately. Emanuel Steward, we miss you, we're thinking of you,'' Klitschko said.
Fans also paid tribute to the late American before the bout when the bell was rung 10 times in his memory. Steward, who was in Klitschko's corner for nine years, died at the age of 68 on Oct. 25.
''It was hard for (Wladimir), very hard,'' said Vitali Klitschko referring to his younger brother's preparations following Steward's death.
The older Klitschko, who holds the WBC belt, said the fight ''was unbelievably brutal. (Wach) just didn't want to fall. He somehow kept standing.''
Sylvester Stallone greeted fans beforehand as part of the promotional campaign for the Rocky musical which opens in Hamburg on Nov. 18.