WIMBLEDON, England -- When you've got it, life's good. Your strokes have more sting on them, errant shots are more easily forgotten, and the road to victory seems freshly paved, free of annoying ruts and bumps.
When it leaves you, often as quickly and mysteriously as it appeared, the storm clouds roll in, leaving a sense that rain can't be far behind. The breath becomes more rapid and shallow, the mouth gets dry, and the feeling of unease builds rapidly, often culminating in defeat.
Eight-time Grand Slam winner Jimmy Connors had it. Venus Williams has it more often than she doesn't. James Blake and Lisa Raymond gave it away at a high cost late in Wimbledon's first week.
We're talking about momentum. And in tennis, where there are no timeouts and rosters are fixed at one, there's no coach or manager to come out onto the field to calm down a player or insert a replacement. What you do with momentum often marks the difference between a journeyman career and the legacy of a Federer, Graf or Agassi.
Says ESPN analyst Patrick McEnroe: "In basketball you can call a timeout or put in three or four new players. In baseball, you can take your pitcher out to try to thwart momentum when you see it changing.
"In tennis, you're all alone. When someone gets on top of you, there's nowhere to hide."
Momentum's a funny thing. Just ask Blake or Raymond. Both were on the losing end of matches that seemed to change from one minute to the next. And both know firsthand how devastating the consequences can be when momentum starts to get away from you.
Raymond, 32, was two points away from a straight-set win Thursday in the second round over three-time Wimbledon winner Venus Williams before her game started to break down. Williams' confidence level rose and took her from the brink of an astonishing upset to a comeback three-set victory.
"She almost played great for two sets ... she was just two points light," ESPN's Mary Carillo said of Raymond. "Unfortunately, we've seen that sort of [reaction] before from both women, at that moment, deep in the second set. And that's all Venus needed. She just galloped away with it once she realized Lisa wasn't ready to win the match.
"I had coffee with Lisa this morning [Thursday]. But I've got a feeling I'll be buying her something a little stronger tonight."
Blake was in a somber mood after his five-set, third-round loss Friday to Max Mirnyi, which dropped his career record in five-setters to 0-9.
Blake's normally reliable first-serve deserted him after the third set, and the 53rd-ranked Mirnyi ran off 12 of the last 13 games of the match to blow the No. 8 seed off the court.
"Once it started going off a little bit, then maybe I started thinking about it a little too much," Blake said. "Unfortunately, it was against the worst guy for it to happen."
The 6-foot-5 Mirnyi, known as "the Beast," is a throwback serve-and-volleyer who has great hands at net and can shorten points to his advantage.
Blake added: "[With] a lot of guys, I'll be able to get in the points off my second serve, get into rallies and kind of fight my way back into a match. He doesn't give you time or chances to do that."
Mirnyi was aware of the shift in fortunes midway through the match.
"To have won 6-1, 6-0 in the last two sets is certainly an unusual thing in men's tennis," he said. "There's a lot of time to make adjustments in a five-set match, and I was fortunate enough to stay calm [enough] to do that.
"When you're winning and the momentum is going your way, it's easy to stay upbeat and energized. When you have some things turn around, everything falls in your play. Mentally and physically, you get drained."
Australian Todd Woodbridge is a former nine-time Wimbledon men's doubles winner and currently coaches Jonas Bjorkman and Thomas Johansson. He's here commentating for BBC Radio and says, "Momentum and confidence go together. I think it's a huge part of the game, particularly on a surface like grass where the points can go quickly.
"It's even more important in doubles because the points are short. When you can get on a really good momentum roll you can turn a match in just a few points."
Woodbridge has first-hand experience with using momentum to his advantage in high-stakes matches at the All England Club.
"I played a semifinal with Bjorkman the first year we won here (2002) where we were down 2-1 in sets and 4-5 down in the semis (to Donald Johnson/Jared Palmer). We turned it around and kept our momentum all the way into the final."
Woodbridge and Bjorkman won the first two two sets, dropping only three games, against the team of Mark Knowles/Daniel Nestor and cruised to the title.
"There can be three or four [momentum] swings in a long match," Woodbridge adds. "Players who can bomb serves, hit tough returns and be really forceful -- once they get it are really tough to stop."
His advice for a player finding himself on the wrong end of a sudden shift? "If it's gone against you, you need to stall and take as much time as you can to extend that period so that your opponent [eventually] goes off," he said. "Instead of playing points quickly, you go longer."
McEnroe knows a thing or two about the subject in his dual roles as a TV analyst and the U.S. Davis Cup captain. He also has plenty of firsthand experience with the phenomenon, coming up on the losing end of a memorable five-set match early in his career under the lights with Connors at the 1991 U.S. Open.
McEnroe was up two sets and 3-0 in a first-round match before Connors, ranked No. 174 at the time and the beneficiary of a wild-card into the main draw, bulled his way back.
"Jimmy was probably the greatest momentum player there ever was," McEnroe says. "He was great at seizing on an opponent's weakness and using the crowd to spur him on.
"The biggest mistake I made was thinking it was over. Once he sensed I was rattled, he sort of smelled blood and got the momentum going."
"Momentum and confidence go together. I think it's a huge part of the game, particularly on a surface like grass where the points can go quickly."
Says Connors: "I had no momentum ... I was trying to figure out a way to get it. When somebody's playing like that and beating your brains out you've got to figure out a way to counteract that. I just figured out a way to get aggressive and play my game."
McEnroe says: "I don't remember being tired, but he was just the aggressor, going for his shots on the bigger points. He wasn't making mistakes. My game didn't completely fall apart, but it came down a notch and then my belief started to disappear."
At that stage of his career, enjoying a final hurrah before he shuffled off into retirement, Connors' game plan was a simple one: "If they were willing to stay out there for five hours and play that kind of [grinding] tennis for that long, then they were going to beat me."
He rode that approach all the way to the semifinals of the '91 Open, eventually adding a classic comeback win over Aaron Krickstein in the fourth round before he was eliminated by Jim Courier.
Ironically, later in his career, McEnroe enjoyed some success in coming back from two-set deficits in Grand Slam play, which eventually helped him look back without regret on the tough loss to Connors.
McEnroe's at ease with the fact that shifts in momentum -- sometimes subtle, at other times seismic -- are simply part of the game.
"It's happened to all of us, losing big leads," he says. "That's part of the beauty of being a tennis player and part of the torture of it as well."
Connors made a career out of using momentum and crowd support to his advantage. When asked how a player gets it -- and keeps it -- he says, "Well, that's what they pay guys a lot of money to figure out.
"I'm not a sports psychologist, but I used it to my advantage. You have to figure out a way to keep that while you're playing, and if you can do that, that's why Borg is Borg and Federer is Federer."
Whit Sheppard is a Paris-based sportswriter who is covering Wimbledon for ESPN.com. He can be e-mailed at email@example.com.