A few years ago, the hedge-fund billionaire Bill Ackman paid $100,000 to play in a charity doubles tennis match against John McEnroe, the former longtime world No. 1 player and multiple Grand Slam champion. Ackman, an indefatigable tennis enthusiast who has toyed with building a court on his office roof, was paired with Patrick McEnroe, John’s brother; John partnered with Craig Effron, another hedgie and the brother of Blair Effron, the co-founder of Centerview Partners, a boutique investment bank, and a close economic adviser to Hillary Clinton. Early in the match, McEnroe hit a lob to Ackman, who slammed it back, directly at McEnroe’s body. The gentlemanly thing to do would have been to warn McEnroe that the ball was coming at him, and then to subsequently apologize. But Ackman instead smashed the shot and turned around and walked to the baseline, without saying a word.

McEnroe seemed pissed. Here he was humoring some hedge-fund guy for charity, and the dude was taking it so seriously and being rude about it to boot. In his younger days, McEnroe might have drilled the next shot directly between Ackman’s eyes, but he instead decided to stick to a more gentlemanly game. Needless to say, McEnroe and Effron won handily.

These days, Ackman and his multi-billion-dollar hedge fund, Pershing Square Capital Management, are getting pummeled by a force even more powerful than John McEnroe on the tennis court: the market. Pershing Square, which had nearly $20 billion under management in March 2015, is down to around $11.4 billion after 18 months of massive losses in its concentrated portfolio of 11 companies, and in particular at Valeant Pharmaceuticals, where Ackman is the largest shareholder. This summer, Fortune reported that investors withdrew $600 million from Pershing Square during the first six months of the year. A source close to Ackman told me that $1 billion had been redeemed over the first nine months of 2016. It’s possible that investors would have pulled more but for the complex rules that Ackman has imposed limiting withdrawals.

Ackman, for his part, claimed on a July conference call with investors that the redemptions were no big deal. “Redemptions were 37 percent lower than the average of the last eight years,” he said. But the Schadenfreude among the hedge-fund crowd is nevertheless palpable. Ackman, a commanding physical presence with a shock of silver hair and intense green eyes, is seen by many of his colleagues as indisputably talented, but also at turns condescending and arrogant. Now some are wondering if his run is over. “I think he hits a brick wall at about 197 miles an hour,” says Marc Cohodes, a Wall Street short seller. “I think he’s done. I think he’s a dead man walking and he’s completely un-investable.” Adds another hedge-fund manager who knows Ackman, “I think he’s proven himself incapable of managing risks. I think that’s kind of obvious at this point.” Notes another hedge-fund manager: “I don’t think that it’s particularly fair, but there is some glee.”

In fact, some hedge-fund managers and investors are now wondering whether Pershing Square has ever made any money for its investors, once Ackman’s fees are deducted. I am told that David Einhorn, the founder of Greenlight Capital and another Ackman nemesis, has done an extensive analysis that shows that Pershing Square investors have taken it on the chin. (Einhorn declined to comment.) Another prominent hedge-fund investor agrees. “I had my guys look at [Ackman’s numbers],” he says, “and if you go through all of it . . . I think he’s returned zero.” (Someone familiar with Ackman’s record disputes this, saying that, for the last 12 years or so, since the inception of the fund, Ackman has made his investors a compound annualized return, net of his fees, of 15.2 percent, as compared to a 7.6 percent return for the S&P 500.)