Milan:Kaka scored in the third minute of extra time on Wednesday to give AC Milan a 1-0 win over Celtic and a spot in the quarterfinals of the Champions League for the fifth year in a row. Kaka surged from midfield and calmly slotted the ball through the legs of Celtic goalkeeper Artur Boruc to eliminate the Scottish champions 1-0 on aggregate. "I am really happy with what I did for my team," Kaka said. "Celtic gave us problems ... but over the two games we deserved to win." The teams drew 0-0 at Glasgow in the first leg of the second round two weeks ago. "I'm very proud of my players," Celtic coach Gordon Strachan said. "The team showed incredible heart. If there was a Champions League for sheer guts and determination, we would be in with a chance of winning it." Milan had plenty of chances to win the game in regulation time but had trouble scoring against a stubborn Celtic. "We knew that it would be difficult but we kept control of the game ... and I think we can be satisfied," Milan coach Carlo Ancelotti said. "Kaka has great speed and is an important weapon for the team." Kaka, who was at the heart of Milan's attacking play, came close in the fourth minute with a curling shot from inside the penalty area that Boruc managed to save by diving to his right. Kaka's 23rd-minute through-ball released midfielder Clarence Seedorf, who directed a pass across the face of goal to Filippo Inzaghi, but Celtic defender Lee Naylor anticipated the move and tackled the striker before his could hit his shot. Maldini rallies Milan captain Paolo Maldini stifled a Celtic attack two minutes later and passed to Kaka, who beat Paul Telfer. However, the Brazil midfielder chose to shoot rather than pass to Inzaghi, who was waiting unmarked in the area. Massimo Ambrosini hit the crossbar with a header in the 38th - although the midfielder was subsequently flagged for offsides. Inzaghi hit the side-netting from close range in the 53rd - three minutes after Austrian referee Konrad Plautz whistled for a foul on Kaka, with Inzaghi in possession and through on goal. Celtic's best chance of the first half came in the sixth minute, when Shunsuke Nakamura's free kick fell to Jiri Jarosik five meters from the goal, but the forward's shot was blocked by Maldini. Celtic never gave up and Maldini did well to cut out Aiden McGeady's pass in the 60th, which was directed at Celtic striker Jan Vennegoor of Hesselink. Maldini also blocked substitute Thomas Gravesen's shot in the 63rd - a minute after the midfielder replaced Jarosik. "The players never stopped believing that we could get a goal," Strachan said. "We've got an incredible will to win ... If we can improve on technique we'll be a good side. That's the vision." Inzaghi and Ambrosini had headers cleared off the line in the 71st and two minutes later the striker was replaced by Alberto Gilardino. Boruc made two fantastic saves with six minutes of regulation time to play - the first on Maldini's 84th-minute header from point-blank range and the second on Seedorf's in-swinging free kick in the 87th. A minute later, Celtic was saved by the crossbar following Kaka's booming strike and the game went to extra time. "It was a fantastic evening," Strachan added. "The night belongs to those who played the game and the supporters of both sides." The game was played in front of a reduced capacity at San Siro after Italian soccer officials imposed strict security measures in the wake of the February 2 killing of a policeman in rioting by fans at a Serie A game in Sicily. About 53,000 spectators entered the 83,600-seat stadium. Celtic fans had been allocated 4,500 tickets for the match but Scottish support at San Siro was at least double that figure.