Venus Williams celebrates a point with teammate Anastasia Rodinova during the Kastles’ match against visiting Boston. (Katherine Frey/THE WASHINGTON POST)

Venus Williams was returning warmup serves on Monday night at Kastles Stadium while a few notable guests were settling in behind her.

A few minutes earlier, first lady Michelle Obama and her daughters, Malia and Sasha, found their front-row seats above the west endline.

From their perch, they watched as Williams returned to World Team Tennis and led the Washington Kastles (6-0) to an easy 20-11 win over the Boston Lobsters. Washington extended its winning streak to 22 matches dating from last year.

Williams was impressive in her pair of doubles wins, but appeared fatigued in her 5-0 singles loss to Boston’s Irina Falconi.

As the match wore on, Williams’s strength and mobility quickly declined. Her serve dropped and she double-faulted five times. Her returns weakened as well, as many failed to clear the net when played from the rear of the court.

“It goes fast in team tennis if you don’t get a hold,” Williams said. “I made errors and she made her shots. I just cooled down.”

The first lady and her daughters left at halftime after the Kastles had convincingly won their first three sets, two by Williams. It was the family’s second trip to the stadium, as they watched Serena Williams last summer.

“That was awesome that they were here,” Williams said. “I had a chance to meet the first lady and the first kids and they were really sweet, nice people.”

Williams started her night with consecutive wins at women’s and mixed doubles. She said the time between her last doubles match and singles affected her play. In mixed doubles, Williams teamed with Leander Paes for an easy 5-1 win over Boston’s Carly Gullickson-Eagle and Eric Butorac.

Williams played strong as she stayed close to the net and exchanged returns with Gullickson-Eagle. With each exchange, the crowd cheered as Williams grunted and mumbled “come on.”

Williams clinched the fourth game with a beautifully placed lob shot that split the Boston players before landing on the endline. Gullickson-Eagle and Butorac turned to each other with a look of disbelief.

Williams then teamed with Anastasia Rodionova for a 5-1 win in women’s doubles. She swept her first service set and won six of her seven first-serve points for the match.

Williams said her health has been a journey since last year, when she was found to have Sjogren’s syndrome, an autoimmune disorder that causes fatigue.

“I’ve come a long way, and I feel like to cover the final stretch, that would be amazing,” Williams said. “At this point, I’m still very grateful that I can still play tennis.”

Williams won the doubles title with her sister Serena at Wimbledon and said the pair has committed to play together more often this year.

Despite her career success, Williams said her qualification for this summer’s U.S. Olympic team was the “ultimate achievement.” In the past, Williams said, she took her appearances for granted.

“I was on top of the rankings, I didn’t have any problems,” Williams said. “But this time, I had to fight so hard for it just to get into contention and come back from so much.”