It took the Grand Slam Committee more than two months to decide on a punishment for Serena Williams after her foul-mouthed outburst at a line judge during this summer's US Open.

Yesterday the committee announced that, unless she transgresses again, the American will not be suspended and will be fined the equivalent of just 1.3 per cent of the prize money she has earned this year.

Williams was ordered to pay $82,500 (about £50,000), which will be doubled if she is found guilty of another “Grand Slam Major Offence” in the next two years, in which case she will also be banned from a future US Open.

This year alone the world No. 1 has banked a women's record $6.55m (£4m) in prize money, not counting her earnings from endorsements and other commercial activities. Her career prize money earnings stand at $28.51m (£17.37m).

The Grand Slam Committee, representing Wimbledon and the US, French and Australian Opens, investigated the case after officials at Flushing Meadows imposed their maximum on-site fine of $10,000 — which will be deducted from the $82,500 penalty — during the tournament in September.

In a tirade laced with expletives, Williams had threatened to “shove” a ball down the throat of a line judge who had foot-faulted her near the end of her singles semi-final against Kim Clijsters.

Williams lost the match and was fined but was allowed to continue playing doubles at the tournament. At the time there were calls for her to be suspended immediately. Williams earned a total of $548,520 (£334,000) at the tournament, $342,825 for reaching the singles semi-finals and $205,695 for winning the doubles with her sister Venus. While the fine is the largest ever imposed on a player for a misdemeanour at a Grand Slam tournament, the committee could have suspended her.

In 1995 Jeff Tarango was banned from the following year's Wimbledon after walking off the court during a match at the All England club and accusing the umpire of being corrupt. He was also fined a then record $63,000 (£38,400). Williams, moreover, could transgress at lesser tournaments without triggering the threat of a future suspension from the US Open. The punishment applies only to Grand Slam events and not those tournaments organised by the Women's Tennis Association for the rest of the year.

Stacey Allaster, chairman of the WTA, pointed out that Williams had admitted behaving wrongly and had apologised for her actions. “I have no doubt that she has learned from this incident and that we will never see her act in this manner again,” Allaster said in a statement.

Meanwhile, Juan Martin del Potro feels he still has a lot of work to do if he is to break into the world's top four.

The 21-year-old can reflect on a year in which he has really come of age, beating Roger Federer to win the US Open and then reaching the final of the Barclays ATP World Tour Finals at London's O2.

A win would have taken him above Andy Murray into fourth in the rankings but he was beaten 6-3 6-4 by Nikolay Davydenko and will start next season 425 points behind the Scot. Del Potro said: “I have to keep working. My goal is to feel better with this part of my game so I can be in a better position.”