On February 7, 1976, Sittler set an NHL record for most points by an individual in one game when he made ten points (six goals and four assists) against the Boston Bruins - a record that still stands after more than forty-five years. He also remains the most recent NHL player to score six goals in one game.

Toronto Maple Leafs

Sittler grew up in St. Jacobs, Ontario, and played minor hockey in nearby Elmira. He was drafted out of the Junior C Elmira Sugar Kings by the London Nationals, soon renamed the London Knights, and played under coaches Turk Broda and Bep Guidolin.

Sittler was selected eighth overall by the Maple Leafs in the 1970 NHL Amateur Draft. He was named team captain on September 10, 1975, after Dave Keon left the team to play in the World Hockey Association following a contract dispute with Leafs owner Harold Ballard.

In his first season as captain, Sittler finished the season with 41 goals and 59 assists, being the first Leaf ever to reach the 100 point mark. A few months later, he tied the playoff record for most goals in one game, with five against the Philadelphia Flyers. That summer, in the inaugural Canada Cup, he scored in overtime to win the final series for Team Canada over Czechoslovakia.

On February 7, 1976, in a game between Toronto and Boston at Maple Leaf Gardens, Sittler set an NHL record that still stands by tallying six goals and adding four assists for ten points (eclipsing Maurice Richard's record of eight, set in 1944). All his points were scored against rookie goalie Dave Reece in an 11–4 Maple Leaf victory.

In 1977–78, Sittler's 117 points ranked him third in regular-season scoring behind Guy Lafleur and Bryan Trottier, and also earned him a Second Team All-Star selection. Sittler's scoring totals remained a Leafs record until being surpassed by Doug Gilmour in 1992–93.

The 1978–79 season saw Sittler suffer some knee problems and miss ten games. It was also the year Leafs owner Ballard fired and then rehired coach Roger Neilson, a process which saw Sittler lobby on the players' behalf for Neilson's reinstatement.

Sittler's relationship with Ballard slowly deteriorated, particularly after Ballard hired Punch Imlach as general manager in July 1979. Imlach and Ballard both had strained relations with NHLPA executive director Alan Eagleson, who, as a player agent, represented more than a dozen Leafs, including Sittler and his best friend and linemate, Lanny McDonald. Imlach believed Sittler had too much influence on the team and tried to undermine his authority with the players. When Sittler and goaltender Mike Palmateer agreed to appear on the TV show Showdown, as negotiated by the NHLPA, Imlach went to court to obtain an injunction to stop them. When Imlach said he was open to offers for Sittler from other teams, Eagleson said it would cost $500,000 to get Sittler to waive the no-trade clause in his contract. So, instead of trading Sittler, Imlach sent McDonald to the woeful Colorado Rockies on December 29, 1979. In response, Sittler ripped the captain's C off his sweater, later commenting a captain had to be the go-between with players and management, and he no longer had any communication with management. Ballard would liken Sittler's actions to burning the Canadian flag.

Through the summer, Ballard insisted Sittler would not be back with the Leafs. But before the start of the 1980–81 season, Sittler and Ballard appeared at a news conference described as "all smiles and buddy-buddy" to announce that Sittler would be at training camp. He showed up with the C back on his sweater, reassuming the role of team captain. Sittler had arranged the talks with Ballard on his own. The discussions took place with Imlach hospitalized following his second heart attack. At the news conference, Ballard said the real battle had been between Imlach and Eagleson, and Sittler just got caught in the crossfire.

During the 1981–82 season, Ballard considered Imlach's health to be too poor for him to continue as general manager. But even with Imlach gone, Sittler's relationship with the Leafs worsened to the point where he told Ballard and acting general manager Gerry McNamara at the end of November he would waive his no-trade clause if he was sent to the Flyers or the Minnesota North Stars. In the first week of December, Eagleson agreed to terms with Flyers' owner Ed Snider and North Stars' general manager Lou Nanne. But it took another seven weeks for the Leafs to make a deal. During that time, Sittler added the Islanders and Buffalo Sabres to the list of teams he could be traded to. On January 5, 1982, on advice from his physician, Sittler walked out on the Leafs, saying he was "mentally depressed" because a trade was taking so long to complete.

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